Peter Griffin, I’d say.
Last night I saw the Family Guy episode “Long John Peter,” in which the fat man, among other things,
murders a man while leading a band of land pirates
I’d say he’s cemented his ranking as Earth’s most evil comic lead. Others may disagree, but I’d want cites. 
Eric Cartman tricked a kid into eating his own dead parents.
Stan from American Dad has killed a lot more people than Peter.
Stan, at least, confines his killing to terrorists (well, alleged terrorists) and other villainous types. And he was willing to beat his boss to a pulp for breaking his daughter’s heart. He’s nowhere near as evil as Petey.
Cartman from South Park easily beats Peter in this contest. Among many, many other examples of his sheer evilness, Cartman has dressed up in a Nazi uniform and tried to inspire Christians overly hyped-up over The Passion of Christ into what he hoped would be an eventual attack on Jews, he pretended to have Tourette’s Syndrome with the aim of getting on national TV and spouting his over-the-top anti-Semitic beliefs (in the guise of being unable to help saying bad things), he tried to get his then-comatose friend Kenny’s life support yanked so he (IIRC) could keep Kenny’s PSP, and finally, an oft-cited example:
In the episode Scott Tenorman Must Die, the aforementioned Scott humiliates the younger Cartman with a series of tricks, like getting him to pay $10 for pubic hair (Cartman didn’t realize how one typically gets it). Cartman sets up an elaborate scheme in which Scott’s parents are killed, and fed to Scott in chili. (On preview, this has been mentioned.)
Except Peter isn’t evil, he’s just stupid and rash. Stewie would be closer, but he mostly just wants to be evil.
Not so much anymore. Now he’s just really, really gay.
Oh, I’d call Peter evil. He’s not MALICIOUS, but he’s definitely agent of Mephistopheles.
That said, I must concede that Eric Cartman takes the top spot.
That said, you guys realize this thread is really just an excuse to list evil acts by sitcom characters, right? No need to stop naming NEW nominees just because the winner is foreordained. 
For evil one should also consider the camouflage that the character exhibits. A misanthropic sort, who is always muttering about the need to kill all the assholes in the world just isn’t hiding things under a bushel. Once his activities become canon, and get noticed by the authorities in his/her world, they will be stopped, and usually within days of the beginning of his/her enlargement.
No, the truly evil character is one who sits in the center of a web of deception, letting people bring their problems to him, for him to solve. And which he will use for the furtherance of his own aims. Whether they have anything to do with the situation that he had been asked to resolve, or not. I am reminded of one incident where the villain I am in mind of used a situation of his own devising to blackmail an innocent into giving up the one thing in his life that gave him joy.[sup]1[/sup] For the simple joy of it, every time he is tasked with solving a situation he does so with malice aforethought to provide evidence that his position is indispensable, and to smear the reputations of everyone else around him.
Jeeves, is, without doubt, or excuse, the most manipulative and evil bastard to ever grace a situation comedy.
Don’t forget the old Etonian spats…
Joe
Does Barney and Friends qualify as a sitcom? Because if it does, I have a nomination in mind.
I think this contest should be split into cartoon, and non-cartoon divisions.
I think you’re describing Sir Humphrey Appleby.
And while we’re throwing out nominations; Louie De Palma.
The Great Gazoo is the most evil sitcom character ever.
Evidence:
He invented a button that would destroy the universe if pressed.
He is The Great Gazoo.
Any more questions, dum-dums?
“Is that the sound of butting in? Why, it must be Springfield’s answer to the question nobody asked, Lisa Simpson!”
How about Dr. Kelso from Scrubs
Mr Burns from The Simpsons
Humphrey’s not evil, exactly. In fact, Humphrey is more amoral. He doesn’t much care what policies he’s implementing, just that his department is run well. As he puts it:
He is, if you’ll excuse the geekiness, a perfect example of Lawful Neutral alignment.
How about Newman from Seinfeld? He was only a mailman, so his chances to do evil were limited, but do you have any doubt about what he would do if he became dictator of the world?
For cartoon characters, I have to go for Zapp Brannigan.
There’s mention of several completely innocent worlds destroyed by him just to further his career and millions of soldiers die during the series due to his incompetence.
Also, he slept with his best friend’s widow while he was still warm. Hell, he wasn’t even actually dead. Now, that’s low.
As for real life sitcoms… I’ll go for Jane from “Coupling”. She lies during her job as a traffic reporter just so she can drive home more easily.
The answer is Cartman, followed by Bender. Everybody else is a pansy.
Oh, please. Bender WANTS to be evil, but there’s just too much he won’t do. For Zeus’s sake, he was about to sacrifice his own existence for a freaking TURTLE, and every time he yelled “Kill all humans!” he’d whisper “Except Fry” when no one was listening.
Also, there’s the time he shot and killed Meg for no reason at all.