The Usual Suspects: When Keyser Soze kills off his entire family, two of the thugs that threatened them, and then let one go in order to send a message to anyone stupid or crazy enough to screw with him.
Return of the Jedi: Uhhh... From the moment that nasty old bastish walks off the shuttle, with the brooding male chorus in the background, we know that this guy's worse than Darth Vader. "Give into your anger. Take your Jedi weapon. Use it! With each passing moment, you become more my servant. It is unavoidable. It is your destiny. You -- like your father -- are now... MINE!" Evil bastish.
The Godfather II: Although most people are pretty ambivalent about Michael Corleone, I'm certain he's an evil bastish (the epitome of LE IMO). After discovering that his only surviving brother, Fredo, betrayed the family, Michael disowns him and tells him people to make sure that nothing happens to him while their mother lives. Fredo, sick with guilt, begs to be let back into the family, but Michael won't speak to him. It takes the death of their mother and their sister to convince Michael to see his brother at the wake. Michael confronts a tearful Fredo and the two embrace, Fredo clearly repentant for what he did and truly committed to his brother. However, we see Michael give one of his lieutenants a look... Afterall, their mother is dead.
X-Men United: Colonel Stryker's in his bunker, viewing the few mutant children he was able to take after his successful assault on Xavier's Institute on closed circuit. He is now ready to complete his goal to kill all mutants. When his henchman asks why Stryker's keeping the children, the Col. replies with a smile that, "I'm a scientist... I like to study the effects of my research."
X-Men United: Magneto manages to stop Xavier from destroying all mutants. Rather than free his old friend, he reworks Cerebro II and tells the hypnotized Xavier to kill all humanity before leaving his former friend to be sealed in a subterranean bunker forever.
There are many more... But the best are those that feature The Joker!
The Joker: When don't we have an "I am evil" scene from him. In The Dark Knight Returns, he kills the entire audience of a talk show (parodied from David Letterman) before beating the hell out of Selina Kyle before serving poisoned cotton candy to boy scouts before scattering bombs around a carnival before committing suicide in such a way to frame The Batman for his murder (in one of the best death scenes in any comic). In A Death in the Family he takes some medical supplies he plans on selling to the blackmarket... but rather than just replacing the real medicine with empty boxes, he switches it with his Laughing Gas... and then he beats Robin to death before blowing him and his mother up. In The Killing Joke, The Joker shots Barbara Gordon (witlessly -- we think -- ending her career as Batgirl), crippling her; then he strips her, takes pictures of her torture, and shows the pictures to a kidnapped Commissioner Gordon. And, finally, in No Man's Land, just before Gotham is reopened, to reflect that there is no hope, The Clown Prince of Crime kidnaps all of the babies born in No Man's Land a few days before, intent on blowing them up. Gordon's wife, Sarah Essen, finds him first and The Joker kills her and gives himself up. When Gordon confronts him and has an opportunity to kill him since Batman won't stop the execution, Gordon can't bring himself to shoot even as he commiserates over the fact that The Joker tortured him, crippled and tortured Barbara, and murdered his wife, The Joker says "Don't you have a son too?" Brilliant.