All rights belong to Akira Toriyama, Toyotarou and Toei Animation
This is based on the manga cannon, so there may be some discrepancies with the anime.
Kakarot did indeed not like Vegeta's plan, for Kakarot to use the mafuba into Vegeta, and conveyed as much at length. Vegeta had not wanted to explain the real catalyst for his decision, currently upstairs probably plotting a new means for his demise. Instead, they had argued for some time on the relative merits of sacrifice and waiting. Until a frustrated Vegeta finally burst out-
"I don't want my family to become like me!"
This caused Kakarot to pause, considering his fellow saiyan as though seeing him for the first time.
"We don't know how long it takes for the evil within them to become permanent." Vegeta growled, "We can't afford to waste what precious time we have arguing."
"But Vegeta," Kakarot reasoned, "We don't know that it will work. I couldn't do it. And we've run out of sensu beans."
"A. You may trust me on this that we are not cut from the same cloth; I will succeed where you have failed, you may count upon it. And B., I would rather die than live like I was before, a creature bent on the destruction and pain of others."
Kakarot seemed to take umbrage to something he had said, however, countering with, "How can you say that you'll succeed?! In case you haven't noticed, prince, I'm stronger than you! I've always been stronger than you!"
Vegeta failed to keep a lid on his own temper, and crumbled under the pressure in much the same way as his partner, shouting against his better judgment and lifelong secrecy, "Because my middle name is Saiya!"
It was a statement that would have sent any of his species reeling. It was a revelation which usually foretold death. But for Kakarot, Earth-raised that he was, it meant nothing.
"So?!"
Vegeta deflated as the gravity of what they were about to do sobered him.
"It doesn't matter, Kakarot. You're just going to have to trust that I am not speaking out of hubris."
A roar of frustration reached them from the house, and reinforced Vegeta's resolve.
"For my wife. For my son. Do it, Kakarot."
And so it was that one of the few orders the prince had ever given his subject would be the one to end the Vegeta the world knew and had come to accept.
Vegeta thought first of the family he had lost, the family from which his heritage derived. He would never properly reunite with them; he would be too damaged, and they would not recognise the beast he was to become. Would his father be proud of him for realising his duty? Or ashamed? Haunted all his life by the thought of his father's mortification, Vegeta found to his astonishment that he no longer cared if King Vegeta disowned him.
He had a new family now. One he cared for more than anything else in the omniverse; more than his life, more than his self.
As Kakarot took his stance, Vegeta looked one last time at the window behind which his wife was brooding, sending out a silent apology to the woman he loved, and to the son she had borne him.
The haze began to thicken around them, coalescing in a hypnotic symphony of hatred. Vegeta shoved down the urge to attack, lowering himself to his knees in a position of supplication.
Of capitulation.
With a cry from Kakarot, his long-time rival turned friend, it began.
And then all Vegeta was aware of was pain.
Goku wasn't sure what he preferred. The incessant, blood-curdling screams of incomprehensible agony had been horrible, that was undeniable. They had wrenched at Goku's heart strings, making him want to rush to the body he couldn't see and comfort his reckless friend. Made him want to call this foolishness to a halt, but he knew that Vegeta would never forgive him for backing down when his family was in need.
But Goku thought the ominous silence which had replaced the screaming was worse. The minutes were dragging by much slower, each second lasting agonising moments. It had to be nearly over; Vegeta would not last much longer.
If he was even still alive.
But he had to be alive, not just for Goku's sanity, but because his mafuba was still working, the darkness was still being absorbed into Vegeta.
He supposed it could still work of a corpse, but he really did not want to consider it.
Vegeta had to still be alive.
And this whole thing needed to end, now!
Finally, finally, starlight broke through the darkness as the last wisps entered a still body.
"Oh gods, Vegeta," he moaned, by the other man's side in an instant, wanting desperately to cradle him but not sure if it would jostle his injuries too much.
And the injuries. They were just – there were no words. Just blood, soaking the grass and assaulting Goku's sensitive nose. He could see bones, a chilling blood-stained white exposed to the night air. Burns littered the skin that was now visible, absent Vegeta's usual armour, which had unaccountably been damaged.
Goku needed to move, though, and fast. He wasn't sure if Vegeta was even breathing.
Would human hospitals be enough, though? Based on his stay after the fight with the figure lying so very still beside him, Goku was inclined to think not.
But there were no more sensu beans so –
Goku gasped, panic and anger mounting in equal parts as he remembered – Dende. And he was angry because he was sure Vegeta had known and chosen not to remind him, giving himself a true hero's end.
Well, that would not happen on Goku's watch.
Breaking his promise to himself, Goku focused on the threads of energy of a being he had not interacted with for some time. Now was not the right moment to reconsider his choices, but he did suppose, as he used instant transmission yet again, that he ought to have at least visited.
Dende was outside when he arrived on the Lookout, leaning heavily on his staff as he contemplated the most recent developments.
"Dende!" Goku cried urgently, "Dende, please! You have to help!"
The smaller namekian was in front of him with a swirl of his white gown, gasping and reeling back as he took in the state of Goku's cargo.
"Please, tell me you can heal him!"
Dende swallowed, putting aside his worries and focusing on the task at hand.
"Put him down and I'll see what I can do."
Goku didn't notice Piccolo approaching them as Dende's hands stretched out and he began the healing process. Warm light began to sprinkle in fine circles over the damaged tissue, and Goku could see the broken skin starting to knit together.
Very slowly.
Unconsciously, his hand reached out to grasp Vegeta's, smoothing his fingers over the back of the prince's hand.
"Goku," Dende's voice broke in, "Is there any chance that he doesn't want to be healed?"
"Yeah, he was happy to sacrifice himself, but I couldn't let him!"
"No, Goku, that's not what I meant. It is one thing to be at peace with death, another to reject the healing process."
"He's not – you cant -?!"
"I can heal him, but it will be slower than usual. He is fighting me. I want to know why. There is something else, but tell me what happened first."
A sudden, strangled wheeze from the floor made Goku jump, his own chest hurting as he realised that no, Vegeta had not been breathing before, contemplated how close his companion had come to dying for a third time.
It wouldn't be good for Vegeta to die. Sure, they could revive him, but how much time would he have to spend in the Other World? Would he go to Hell? Would he suffer endlessly until they managed to collect the dragon balls? The namekian ones, he thought.
Would he even agree to come back?
"Goku?" the young god prompted.
"Oh, yeah. Well, you know that the um, we call it the Great Darkness, has been on Earth."
"I am aware, yes. It seems to be inside your friend, now."
"Oh, you can tell, huh? The mafuba didn't work, so we saw how it was imprisoned originally. This guy, Keel Vegeta, he sealed it inside himself, but the process killed him. We brought another version of the Darkness back with us from the future, though. That's how it ended up here. Long story short, I used the mafuba technique to seal it inside Vegeta. I think he fused with it or something. I'm not really sure. It didn't work on me when I tried it."
"I see. And the sealing or fusing process, whichever it was, caused these injuries?"
"Yeah."
"Why would he not want me to heal him?"
"Ah, well, you see, we think that the Darkness would…that is to say…it's all inside him now, right? He didn't want to live like he used to, all evil and stuff."
"That's noble." Piccolo broke in, looking at Vegeta was a new sense of appreciation.
"So you believe that this Darkness inside him…will make him malicious when he awakes?"
"We thought so, yes."
"There is something you should know, Goku," Dende said quietly, gesturing for Piccolo to leave them.
"When the Darkness first showed itself on Earth I was already familiar with it." He continued in a low voice. Goku nodded patiently, not seeing where this was going.
"You see, I had already encountered it when healing Vegeta on planet Namek."
"Huh? What was it doing on Namek?"
"It goes wherever Vegeta goes, Goku. It was inside him. It still is, only there seems to be double the amount now."
"I…don't understand."
"Vegeta already had the Darkness inside him, Goku. The version you brought back from the future was probably inside his future self as well."
"His future self was dead. Does it escape when you die? I guess I'll have to ask Whis. I kinda thought it didn't. But you know, Vegeta having the Darkness and all, that actually makes sense. But it does give me some hope, you know?"
"How so?"
"Vegeta was kinda…okay, really evil when we first met, but he's a nice guy now. Maybe, with time, having a bit of extra Darkness in him won't affect him all that much."
Dende sighed, "I think you are being overly optimistic, there. This is not a 'bit' of extra Darkness. This is double the dosage, so to speak. And it took years for him to reach the point he is at now, or was before this episode began."
"He can do it," Goku insisted, "I know he can."
"I hope you are right."
The only warning Goku received was an animalistic growl, before Vegeta suddenly flung himself at him, long, vicious incisors snapping.
"Vegeta, what?!"
He dodged around the flailing punches, taking in the crazed look on his friend's face.
"What is wrong with you…?" he said to himself, ducking underneath a strike and tackling Vegeta around the stomach.
"Calm down!" he ordered, only for Vegeta to jump to super saiyan, escaping his hold. He howled at Goku, screeching like a wild banshee and hurling punches at the other saiyan with reckless abandon.
"Stop it!" Goku yelled, shoving Vegeta awaay and trying to assess the situation. Vegeta retaliated with a highly-charged ki blast aimed at his face.
"Stop it I said!" Goku dashed behind the prince, grasping him tightly and shoving him onto the damaged tiles. The other saiyan squirmed in his grasp, but was ultimately helpless.
"Has he gotten weaker?" Goku questioned no one in particular, but Dende answered anyway from his hiding place behind a pillar.
"I believe so, yes."
And Goku felt like crying, then and there, just shedding his tears for all to see because he finally comprehended the magnitude of what his friend had done. What he had agreed to. Not only his life, not only his self, but his very strength too, his power which meant so much to him.
How could Vegeta have been willing to give that up?
And how could Goku help him?
"All right, I'm taking you to Whis," he informed the wriggling, growling madman beneath him, "Maybe he'll know what to do."