Note: This story mostly follows the manga, when Zamasu never actually met Goku and only watched him fight during the initial tournament organized by Beerus and Champa. It's an exploration of Zamasu's psyche right before he murdered Gowasu.
He saw himself floating above a ruined city, arms wide open as in an embrace of the chaos below. Behind him, a blinding circle of light shone like ten thousand stars; a radiant halo of splendour, of unfathomable justice that transcended time and space. He shivered with joy, tears streaming down his godly face as he contemplated the burning pyre of rebirth, his mind infused with endless knowledge, and his body replete with power.
Zamasu emerged from meditation, his brain glowing and inflamed with murderous intent. Senses heightened, he assessed his surroundings, not immediately recognizing his room. As he brought his thoughts into focus, he distractedly picked up one of the mutilated books that lay open beside him, and discarded it when he realized that most of its pages were missing. He remembered the lies he had torn away from the binding, which reminded him of his location, and of the duties that awaited him. Grimly calm, he uncrossed his legs and rose from his cushion, then carefully made his way to the sliding doors, stepping over the frightening number of books, teapots and cups of unfinished tea that littered the floor. Though he did his best to avoid every object, he accidentally knocked over one of the pots, and watched intently as the straw mat absorbed the bitter drink, wondering if it would absorb blood the same way.
He stood for a moment in front of the round paper doors, and opened them slightly to let some light in. A cool rush of air scattered the pages at his feet. Feeling the warmth of the sun on the left side of his face, Zamasu peeked into the stone garden outside, but images of the mortal he had seen in battle soon flooded his consciousness. Watching him fight had been like a revelation. Never before had he felt such awe, such astonishment, mingled with rage.
This is the power of a mortal?
He dwelled on that limitless body, wasted on a mere Saiyajin, but did not welcome the envy that surged through his veins. He managed to suck out the poison before it could intoxicate him completely, but his anger was harder to subdue; a wretched demon that struggled incessantly to liberate itself from the confines of his morality. Despite all his efforts, it was always there, lurking, hidden in his innards, behind his façade of temperance and composure.
Zamasu slid shut the doors, finding himself more comfortable in the darkness. He went into the next room and gathered all the necessary implements to make tea. He selected the leaves his master loved most, warmed the kettle, and filled it with water, which he boiled with the energy he gathered in his palm. After placing everything on a cart in the corridor, he put the leaves in the infuser and added water. With precision and diligence, Zamasu timed himself, and dunked the infuser a few times to circulate the water before removing it and setting it aside. Then he covered the pot and left at last to join his master. As he pushed the cart through the long, dimly-lit passage designed for introspection, he noticed that his hands were not shaking at all.
He found Gowasu in the inner courtyard, sitting at the red table with his eyes shut, contemplating. The old master seemed serene, completely at peace with the imperfect world that tormented his apprentice so. Still, Zamasu's heart warmed as he beheld his teacher, exactly as it did on the day when he was first invited to the realm of the Kaiō-shin.
After dedicating the entire morning to training, Zamasu bathed and changed into some fresh clothes. He put on his trousers and his black robe, the one that was brocaded with the Kaiō symbol, and tightly wrapped the sash over his stomach. He drank some water and had a frugal meal, then selected a string-bound book from his library before heading back outside. For hours he read, strolling distractedly through his garden, in endless, perfect circles.
Suddenly, the wind picked up, tousling his white hair. Zamasu lifted his eyes from the page that had captured his attention until then, and saw Gowasu standing before him with a benevolent smile.
"Gowasu-sama!" he cried, bowing almost instantly.
But Gowasu returned the courtesy by bowing as well, much lower than he needed to. Zamasu could not account for this gesture at all.
"Hello, Zamasu. Forgive me for interrupting your studies. May I take a turn through the garden with you?"
"Certainly!"
The two of them walked, side by side.
"How have you been, Zamasu?"
"I've been well, thank you. I've been reading the books you recommended, and I'm enjoying them very much. I hope to absorb all the beautiful teachings they contain."
"I'm pleased to hear it. Which one are you reading now?"
"The one about humility," Zamasu said, showing him the volume he was holding.
"And what have you learned so far?"
"That true humility lies in detachment."
They pondered these words in silence, then resumed their conversation.
"Zamasu, how do you watch over an entire universe while being detached from it at the same time?"
The young Kaiō stopped, displaying a slight frown. He considered the question attentively, but lacked the wisdom to respond.
"I do not know," he humbly admitted.
As though he had been expecting this answer, Gowasu turned around and held out his hand to him.
"Why don't you come to the realm of the Kaiō-shin and learn with me?"
Zamasu blinked, so taken aback that he dropped his book.
"M-Me, go to the realm of the Kaiō-shin?"
"Yes, Zamasu. I would like to make you my apprentice. Become my successor, and this universe's next Kaiō-shin."
"I'm afraid I'm not worthy enough," Zamasu stated, lowering his head.
Was it humility, or instinct?
"The fact you just said that proves that you are."
Their gazes met.
"I…"
"Do you accept my offer, Zamasu?"
"It… It would be an honour!" he exclaimed, unable to contain his happiness any longer. "Master, I will do my best for universal peace!"
He bowed, and his hair fell forward, releasing the scent of aloeswood and incense.
"I choose you, Zamasu, because you are the strongest, most hard-working Kaiō-shin I have ever met. Your skills in martial arts are unparalleled, and you wield the power of the gods with grace and elegance. Furthermore, I believe your heart is pure, and ready to receive divine teachings."
"You honour me, master."
"Please watch over me, as I will watch over you," Gowasu requested with a bow, and the master showed his new apprentice the way.
"You are right on time," Gowasu said, smiling kindly at Zamasu as he drew closer with the clinking cart.
Zamasu smiled back.
"I've brought you your tea, master."
"You've always been so punctual, Zamasu. And incredibly organized."
Zamasu thought of the mess he had left in his room. His stubborn, obsessive ambition to prove that his soul was still fragrant, pure. His rage at the teachings he now found disappointing.
"I hope you like this tea, Gowasu-sama."
I've been practicing, you see.
As he poured the tea from the pot, his hands were as steady as ever. He carefully picked up his master's cup and placed it on the table before him, then took his own cup and sat across from him.
"I am looking forward to drinking your tea today," Gowasu declared.
Zamasu couldn't understand why. The last cups he had served him had not been good at all, and he had clearly seen the grief and concern in his master's eyes.
"I've been looking forward to serving it to you," he replied, sickened by his own sincerity. Quickly, he sought to relieve his guilt with a lie.
"Gowasu-sama," he began, crossing his hands on his lap. "I think I may have been mistaken after all. I thought about what you told me the other day. About the balance between good and evil."
And I will bring balance to the universe.
True balance.
"Did you really?"
"Yes. And here is what I have concluded," Zamasu replied, rising from his chair and walking over to stand beside his master. "Because there is evil, justice is born. Good becomes visible when it is contrasted by evil. My problem is that I have been looking at things from only one perspective."
"Zamasu, I believe that one day, you will understand what I am trying to teach you."
"Undoubtedly."
Gowasu picked up his cup and blew on it, waiting for the tea to be cool enough to drink.
"Of course," Zamasu couldn't help adding, "I have yet to come to terms with a god's duty of simply watching over mortals. I admit that sometimes, I find it painful."
"How so?"
"It pains me to see mortals act the way they do. The world is so beautiful, and still they sully it with their evil."
Still they desecrate it with their very existence.
"It isn't easy, you are right. But that is the burden we must carry."
"What of justice?"
"Zamasu, it is just that we should watch over mortals."
"And do nothing against their evil?"
"We've discussed this before. We cannot—"
"Do you not think that the beauty of the world is lessened by them?"
"Lessened?"
"Master, there comes a time when one realizes that certain things are superfluous. I used to pour sugar and milk in my tea, but I learned to appreciate the basic essence of the leaves, and did away with anything that lessened their taste."
Gowasu chuckled.
"Surely, you don't mean to compare mortals to sugar and milk?"
"No," Zamasu sternly replied.
"Then what are you trying to say?"
"The world holds a basic beauty. As gods, we recognize that beauty. Is it not our duty to protect and uphold it?"
"Of course it is, but… Mortals are part of that basic beauty. Their existence is inherently beautiful."
"I disagree."
Zamasu tucked his hands behind his back, waiting patiently for his master to have a sip of his tea.
"We've spoken of justice, but what of love, Zamasu? Can you not find it in you to love something that is imperfect?"
"Do you love mortals, Gowasu-sama?"
"We must love them in the same measure as we love the beauty of this world."
"How? How can you love them, when all they do is bring chaos to the world? How can you love something so corrupt, so worthless, so vile? "
Corrupt, worthless, vile.
"Zamasu…"
"Whoever allows them to thrive is complicit in their chaos. Mortals are nothing but weeds that must be uprooted. Their lives are of no consequence to us gods. Why should we love them, watch over them and forgive them again and again for their trespasses? They deserve nothing, because they are nothing!"
Uneasy at his pupil's vitriolic outburst, Gowasu flicked a glance in his direction.
"Don't say things you don't mean, Zamasu."
"I mean everything I say," he retorted in a steady, controlled voice.
"Sometimes, the things you say paralyze me, because they don't seem congruous with who you are."
"And who am I, Gowasu-sama?"
Who do you think I am?
"For a while, I thought you might very well be a saint. But in time I realized it wasn't the case. You need not be one in order to succeed me, but…"
"But what?"
"Zamasu, the truth is, there must be something lacking in you if you cannot love those who are imperfect."
"Something lacking in me?" Zamasu scoffed.
"You are an impressive warrior, to be sure, and a brilliant scholar. But can you look deep down within yourself, and say that you are generous enough to love mortals?"
Now he responded with silence.
"I hope in time I can change your mind," Gowasu said, staring into his cup. "You still have so much to learn."
The other god sneered.
Will you wait a thousand years?
Eying the Potara dangling from his master's left ear, Zamasu reminded himself of what he had set out to do. But he could hardly reconcile his design with the affection he still felt for this oblivious old god who so trustingly drank the tea that he served him daily, despite its bitterness, despite the fear and doubts clouding it and compromising its flavour. This kind and compassionate teacher, this accursed fool who forgave his student just as easily as he forgave the folly of mortals.
Your patience is nothing but cowardice.
Fighting the urge to tell he that he loved him, that he hated him, resisting the need to embrace him in his arms and to press him against his heart, Zamasu gathered all his energy into his palm. As he peered into nothingness, his hand became a dagger, a sword, an instrument of treason and divine justice. His eyes gleamed darkly as he rose his arm above his master. He, the purest of Kaiō-shin, the one who had been chosen for his strength, and the perfection of his virtue.
Gowasu brought the cup to his lips, tasting the tea at last.
"This tea is exquisite," he lied, believing in his pupil as fallaciously as he believed in mortals.
Zamasu could find nothing to say in way of an apology, and confirmed the insufficiency of his love by lowering his arm, and beheading his own master.
Having finally crossed the forbidden threshold, Zamasu stood there for a while, satiated, watching the swelling blood as it conquered the spilled tea and began to drip off the table and onto the sacred ground below. There was no turning back now. He looked down at his light blue sash, the same one he had tied so proudly around his waist on his first day at the temple, and realized with indifference that it was ruined. Checking his hands, he saw that they were still steady, so he reached for the head that lay before him like a food offering at an altar, and gluttonously ripped the coveted earrings out of the old man's ears.