I've finished GTA V a few days ago, and holy shit, what a game.
Anyway..... back to the title of the topic, Trevor is a fun character, so fucking insane!
There should be a photo of Trevor in the dictionary, when you search for the word psycopath.
I wonder what other interesting psycopaths you can find in games.

Batman Arkham series ^_^

Malkavians in Vampire the Masquerade Bloodlines
Adam in Cat Lady (has several other psychopaths as well but I thought he was the most interesting)

Dota 2 players including me are all psycho.

I'm not in a game yet, but give me time.I'm getting there.

Racter - Shadowrun Hong Kong.

A more serious answer than my last:
I've never played the game, but I hear Psycho Mantis from Metal Gear was something else.

Vaas - Farcry 3?

Baldur's Gate 2: Jon Irenicus

I think the Dark Savant (Wizardry 7/8, maybe 6?) would qualify. Also, I believe Wizardry 8 has one ending in which *your* party could be considered psychopaths.

Actually, it is not hard to find villains in story-oriented games who are psychopaths.

Edna from Edna and Harvey, maybe?

Probably.

Raiden from Metal Gear Rising: Revengeance.

I really disliked Trevor.

But I assume Manhunt would have at least one psycho in it somewhere.
And Borderlands has a lot of psychos.

Explanation of Trevor!


http://www.cracked.com/video_19346_7-ways-gta-v-makes-fun-its-own-fanbase.html

Just a few titles I remember. No spoilers!
The 7th Guest - the owner of the mansion, Henry Stauf.
Blackwell - quite a lot of psychopath characters.
Deponia - Rufus. Maybe not so visible in the first part but later on… ugh…
Downfall - play the game and you will definitely find a psychopath there.
Dungeon Keeper - Horned Reaper, nuff said.
Warlords Battlecry - evil path is very psychopathic, it’s really worth to play it!