The shame in Knives' voice when he admitted he'd lost control - it had alleviated Vash's fear that his brother would repeat that crime. He couldn't say THAT out loud.

"I- Knives, you and I, we've both been pursuing one goal for our entire lives, and that's over now. We've spent so long putting our hearts into that, and we have nothing to invest that in anymore. That's what I see."