Every night he dreams of burning.
His world, every waking moment, is viewed in tints of black and red. These are his colors: scarlet, and black, black like the void of space without even the stars shining through. These colors are all he knows now. He remembers, as though it were someone else, that once the world was not this way, that once there were shades of cool greens and blues, and pure whites, and brilliant yellows, and the shaded skin tones of a woman—brown eyes—
("You turned her against me!" he screamed.)
--But now the crimson is all he knows, the crimson of rage, of passion, of unbridled fury and hate, hate that he lashes out with to bend the very fabric of things around him. A festering, scarlet hate. And he dreams of burning, the white-hot lava, the tearing agony where his limbs used to be and are no longer, and the stench of sulfur. The stench of his own flesh. Burning. Writhing in agony on the scalding sand, as hot as the fury inside him.
("I hate you!" he'd screamed, spitting it out with all the venom he could muster. Trying to claw his way up the bank away from the burning sand with the one arm he'd had left, his mechanical arm, the heat and the pain unbearable already. And then, there had been a thwump as his pant legs had caught fire…)
When he wakes his windows shatter: he draws on the Force and uses it to spew out the hatred inside him. He hates everyone: Obi-wan for destroying his life, for mutilating him and leaving him there in flames to burn to death, for taking away everything and everyone he loved. He hates Padme for turning against him, betraying after he'd given up everything to save her, he hates the Jedi for lying to him and using him, he hates Palpatine for twisting him and tearing away everything good in him, until he is left with nothing but the Dark Side and the world viewed in shades of crimson. Flames and passion the colors of a Sith. Behind his mask, behind his face, so scarred it hardly looks human, he rails at the entire cosmos. Every time he moves, walks on the prosthetic legs, uses his prosthetic arms to wield a lightsaber, he curses the universe, he curses Obi-wan, he curses the Force. Yes, he curses it. Without it he would have grown up with his mother, and would not have to dream of her bleeding and dying in his arms, and he would not have lost Padme, and he—
But the Force curses him back, and even while he uses it to wield his power, it drains him. Each time he taps into it it takes away a little more of his humanity, a little more of his last remaining goodness. He feels more machine than man now. What is he? Nothing more than a conduit for all the evil that Palpatine and the Force can pour into him. He doesn't mind it so much anymore, it seems. He knows that one day, when he is stronger, more experienced, he will have his revenge on his master. Someone else will come along, someone else with the Force strong in him, and when that happens he, Darth Vader, will bend him and twist him and use him against Palpatine, and together the two of them will be able to overthrow him, destroy him, make him suffer as he has made the entire galaxy suffer. By now he's quite good at spewing out his hate, and everyone he kills is only practice—for when he finally confronts his master and makes him pay, makes him pay for taking him away from the burning sands and making him into this. Each time he takes a breath with his augmented lungs and hears the air rasp in, hiss out of his mask, it reminds him that he still has a purpose. That purpose: kill his master. Before that: make him suffer.
He can wait, of course. He bides his time, obeying his master's every word without murmur, hiding his true feelings. At one time, he remembers vaguely, he would have been appalled at such deviousness. But no: that man is gone now. The man who was Anakin Skywalker burned to death in the flames on Mustafar, and the man in his place is stronger, more dangerous, infinitely more powerful, because all his scruples were burned away and now he will kill anyone he pleases, because he can, because it sates his rage. All the coolness in him was burned away, and now all that is left is black and red. The colors of fire, the colors of Sith.