Alex Mercer from "Prototype". He can disguise himself as anyone, making it really easy to catch one off-guard; add to that the psychological horror upon realizing that the person he's disguised as is someone he ATE! On top of that, he has countless ways he can kill you, be it through simple crushing, slashing you up with claws, dicing you up with a sword arm, piercing you with spear-like tentacles, and more. Worst of all, when he kills you, it is highly likely that he will consume you as well, absorbing your very mind. Who knows what kind of fate that has on the deceased? Is their very conciousness permanently assimilated into this amalgamation of deceased minds, essentially robbing the person of their very soul, ever reliving their own death and unable to pass on in peace? It's... too horrible to imagine...