Monster / Video Games
aka: The Elder Scrolls
"This is the time of vengeance, and no life is worth saving. And I will put in the grave as many as I can! It's time for me to kill, and it's time for me to die! My genocide crusade begins here!"

"Seriously, I can't believe you just asked me that! OK, all right, fine! How about this reason? Seems as good as any. I do all the wonderful things I do because I want to see the miserable look on the faces of people like YOU when you're wallowing in despair, dismay, grief, frustration, misery...all sorts of other unpleasant nouns... I guess you could say I'm bored. At least misery is interesting."

You're really going to be glad when you defeat these characters in your Video Games...unless it's the rare occasion where you're playing as them.
Examples from Visual Novels can be found here.

The following have their own pages:

Other examples:

Alternative Title(s): Halo, The Elder Scrolls, Metal Gear