Re: How do you get players to role play in genre?
I'm reminded, as I so often am, of an episode of Batman: The Animated Series.
A guy shows up in Gotham (background story snipped) calling himself Lockup. He's brutally tough on criminals-- uses unnecessary, life-threatening force, and so on. So Batman confronts him, and it turns out all he wants to do is work with Batman to fight crime; he just doesn't think Batman takes it far enough. Naturally, they have a fight, Triangle wins, etc.
However, I suggest having the PCs encounter a Lockup-type vigilante who goes so far as to kill on a regular basis. The PCs are fighting some baddies, and Too Much Force Man shows up to help, but starts slaughtering thugs, killing unconscious opponents, and generally being an asshole. What do the PCs do? Do they set him straight? Are they appalled by his behavior? If so, then they will have turned a corner, and you can probably continue to steer them away from meaningless violence into less-meaningless violence.
And if they think Too Much Force Man is cool, pack your bags, throw in your five bucks for pizza, and get the hell out of there.