The distinctions are getting finer and finer here. Are we talking about the character who [ol]
[li]regularly carries out the most evil deeds,[/li][li]is capable of the greatest evil, even if they do not in fact carry it out, or[/li][li]envisions themselves as capable of the greatest evil, even if they are not actually capable of carrying them out?[/li][/ol]
We are talking about the character who
- is the Great Gazoo.
Before she had kids and her character softened somewhat, I would say Jordan Cox from scrubs. She was my hero.
Another vote for Eric Cartman. Don’t forget he also deliberately infected Kyle with AIDS.
During the Seinfeld finale, when Newman says something to Jerry along the lines of “You’ll see me Jerry, sooner or later you’ll see me in all my Glory, and then you will be shamed”, I was absolutely certain that the last shot of the series would be a pan-back of Newman in full devilish regalia (replete with horns) laughing maniacally as our anti-heroes are carted off to jail, with the implication that they are in Hell, but (a la the Sixth Sense) they don’t know it. I was very disappointed when that line didn’t pay off.
Brannigan isn’t willfully malicious, not in the way Bender is. Can we still call someone evil if they are unaware that they are committing evil? Heck the Head of Nixon is much more deliberately nefarious.
Since I started the thread by nominating Peter Griffin, the answer is obviously yea.
Evil done to Meg doesn’t count. Ask Lois. 
Dude, I spent something like 5 years believing Nuovo Vesuvio was bugged. Talk about disappointed.
I nominate the full cast of Seinfeld, then. They cause irritation and pain to all they encounter.
Ross Geller from friends. Spent years stalking Rachel Green, a woman waayyy out of his league, and then knocked her up. In the meantime, he taunts her by saying her name during his marriage ceremony to another woman. He turned his first wife into a lesbian. Oh, and he mostly ignores his son. He takes naps with Joey and uses his position as a professor of paleontology to score with a hot black chick, just to jealously mess with Rachel. He framed his best friend for smoking pot so he could deceive his own parents.
And all that was just one episode.
I’ll toss out a couple of oldies for argument’s sake.
Frank Burns from “MAS*H”
Dan Fielding from “Night Court”
Maybe not evil, but they sure were assholes.
And when he couldn’t sell a truckload of aborted embryos for medical research, he tried to unload them as jumbo shrimp.
The Second Stone: How, exactly, is a nap with Joey an evil act? 
Bender isn’t even in Zapp Brannigan’s league. Bender’s conscience always holds him back from the worst. Brannigan is an actual sociopath.
So he wants every human in existence except one dead, and that’s not evil enough for you? Come on. What about when he gave his kid to the Robot Devil?
No, Zapp is just an idiot. He threw wave after wave of soldiers at the killbots so they would reach their pre-programmed kill limit. It didn’t occur to him that it was murder.
Sleeping with Joey Tribiono is just wrong and evil. Seriously, that was what you picked out? You found everything else self-explanatory?
Frank Burns was more incompetent and immature then evil. The CIA agent (whose name escapes me) from MASH might be a good candidate though.
I still think Cartman wins though, assuming we are only counting characters that were created specifically for a given show (otherwise, Satan wins, having been a reoccurring character in several sitcoms, and being, well, Satan)
Flagg?