Villains who are either not evil in their original source material or are more vile than what they are in their original versions.
Sometimes, they do not redeem themselves in the adaptation, with Kurse, Sentinel Prime, King Stefan, and Bolivar Trask being examples; while sometimes they may redeem themselves to get close to what they are in the source material, Darkrai and Hades being good examples. Also note that not only does this apply to fiction; it can also apply to myths and legends like most versions of Hades who have been depicted as more evil than his original version.
Editing note: do not include real life people for they are only included in Fictionalized versions of true events, unless the real life person is a villainous parody of an original adaptation (i.e. Hermann Fegelein (Downfall Parodies) & Queen Victoria (Black Butler)) for instance.