I think that if Vash wasn't around, Knives wouldn't be either. I beleive that what Knives was doing was to impress Vash, and to make himself feel important, and be the... superior 'mutant', if you will.
So if Vash weren't around, who would Knives have to provoke? Who would Knives have to have... fun with? I think he'd just decide that they're too insignificant to worry about, force them to build him a space ship back out of there, and find somewhere where there's someone to challenge.
So, in the end, it's all Vash's fault. He's... EVIL!
Get MY music at http://artists.mp3s.com/artists/331/dj_fromlo.html
This is much clearer in the manga, but Knives and Vash were "Plants", the power source the humans use. Knives was toying with Vash, but his ultimate goal was to wipe out humanity and free the "Plants". He wanted Vash to be by his side for this occasion, and that is why he kept trying to provoke him to kill. The road Vash walks is a hard one and the series does a great job showing both the pros and cons of his philosophy.
"Words to memorize. Words hypnotize. Words make my mouth exercise. Words all fail the magic prize. Nothing I can say when I'm in your thighs."