Because you like The Janitor in Scrubs

A self-professed psychopath, James kills small animals for fun. After getting bored of that, he is ready for an ambitious murder. See them in The End of the F World.

The vampire that killed Zero Kiryu's family also infected him, and he starts to crave blood, much to his horror. See them in Vampire Knight.

Sylar's victims are found all over the country, many with their brains removed. See them in Heroes.

Good-natured, sensitive, and introspective, Mason is on a lifelong path to finding himself. See them in Boyhood.

His superiority complex coupled with his bitter desire for revenge makes Vegeta a formidable foe. See them in Dragon Ball Z.

With an A.I. as loyal and witty as Cortana, who needs humans? See them in Halo.

A fight every now and then makes Dante's life a little more interesting. See them in Devil May Cry.

Lelouch vi Britannia is a high school student by day, masked destroyer of tyrannical empires by night. See them in Code Geass.

Once a lonely hat maker, Sophie Hatter is now cleaning goo from bathtubs and cooking breakfast for the grumpiest wizard in all the land. See them in Howl's Moving Castle.

Keren Walker is a socialite looking to avoid work, keep a constant buzz, and be as snarky as possible. See them in Will & Grace.

There's nothing like discovering dear old dad is a serial killer to send you into an (understandable) funk. See them in Hannibal.

Severus Snape is a misanthropic potions teacher with a vindictive streak. See them in Harry Potter Series.