Disclaimer: I don't own Trigun.
Heads and Tails
When they had been younger, as little boys lost in the sea of stars, Knives had really loved him. Knives had loved everything. He had been hopeful and cheery and… just as much a maniac. He chose something, and he threw his whole heart into it, he trusted it, loved it, protected it and loved it. Knives was either delightful and boyish, or depressed beyond consolation. Any one disappointment could tip his scales or swing his moods. He had always been just a little bit insane.
Vash never liked to dwell on thoughts of his brother because of the pain they brought, but he'd had quite a few years to get used to the pain. An he had always been just a little masochistic. At thoughts of Knives he grieved the brother he had lost, the thousands of people gone because of him, and the fact that really, everything could have been completely vice-versa heads to tails black and white different.
Because Vash had always been the same.
Putting everything in him in support of one idea, into one project, into one pursuit. To trust and have faith in. Vash could have just as easily been the one driven over the edge of his mind by Tesla, by the terrible, insatiable, inhumane, unethical curiosity of humans. Vash could have been the murderer.
Only, Knives had let himself be swayed from loving humans to hating them, to believing in freedom and superiority. Vash just believed in Rem. And he hasn't been let down by her ever.