It's been awhile since I seen the movie but Jack Palance in "Shane". IIRC, he tosses a gun at the feet of some dirt farmer, who won't pick it up even after being taunted. When he finally goes for it, Jack easily outdraws him, killing him. His response: "You all saw it. He had a gun."
Sticking with Westerns, I've also a fondness for the evilness of Gene Hackman in "The Quick and the Dead".
However, one of my all time fave scenes of villainy was an Imperial officer dropping dead and Vader saying "Apology accepted Captain Needa."
But an honorable mention goes out to Mel Gibson in "Payback". Killing a man's alligator bags and shooting holes in his suits? That's just MEAN!