Vash makes the point that people are easier to manipulate into doing what you want than they are to motivate to do the right thing. He believes that people are inherently greedy and self-serving; there's a reason the Jedi have to train so hard and Light Siders without any official Jedi training are a rare find.
He also thinks that the Dark Side makes him stronger.
"We have an advantage over the Jedi," Vash says. "Our fears, our rage... our emotions empower us. They don't have that."