Deadpool - Of course he isn't always a bad guy but when he is, nobody does it better(or worse depending on how you look at it).

Thanos

Juggernaut

Deathstroke from the DCU: Online Trailer - I hate Batman although Arkham Asylum and this version are cool looking, so anyway I loved it when he killed/was in the midst of killing Batman via stick.

Lex Luthor From DCU: Online Trailer - He killed Wonder Woman and the coolest looking Superman ever...