Vash felt sick horror in the pit of his stomach. That a being capable of so much destruction 'couldn't help it?'. Much of what Vash knew about the powers of plants, Knives had presented to him in the most traumatizing way possible. Vash had always been a little afraid of his brother after the Big Fall, in the same way, perhaps, that Knives was afraid of humanity.

This visceral, knee-jerk fear possessed him for a moment, and he couldn't the twinge that passed across his face. Then he fell silent, regarding his brother's injured frame, deep in thought. Despite the fear, they were brothers, after all, the only two independent plants on Gunsmoke. Better than he would ever admit, Vash knew the dark side of humanity, and understood the temptation to do away with them.

Ah. To continue his crusade against them here would be a petty and pointless thing for Knives to do, wouldn't it? Vash didn't even need to think any further.

"I see." he said finally. "Thank you."