Here’s the challenge: find references to characters in TV shows and movies that are professional or noted amateur cooks - who are also killers. Good guys or bad guys, doesn’t matter.
For example: in Under Siege, Steven Segal plays a navy cook … a former S.E.A.L. … who kills a whole wack of terrorists.
Any other examples of chefs who dish out death as well as hors d’oeuvres?
Edit: answers are for a little project for a chef who is a friend of mine …
Would Sweeney Todd count?
Silence of the Lambs
Well, Mrs. Lovett is the cook … I guess she counts. Did she actually kill anyone, though? 
… with fava beans and a nice Chianti.
He didn’t succeed, but Chef Louis (Reme Auberjonois) in The Little Mermaid clearly had crustaceacidal intent. It’s no excuse that his job is killing sealife. Steven Segall operated the same way.
There was an episode of Archer where the celebrity chef was in the pay of the Russians and poisoned someone that Isis had been paid to protect. Actually, now that I think of it, there were two episodes where the cook killed someone Isis was protecting/trying to impress.
Not exactly relevant, but mention must be made of the book Someone is Killing the Great Chefs of Europe (one of the earliest of the mystery-with-interspersed-recipes) by Nan and Ivan Lyons, and the movie made from it Who is Killing the Great Chefs of Europe?* from 1978. It turns out that no murderous chefs were involved (I won’t say who the actual murderer(s) was(were)), but certainly they came under suspicion.
I should also mention the episode of Alfred Hitchcock, “Lamb to the Slaughter”, based on the Roald Dahl short story, in which there definitely IS a chef with murderous intent (and action). Notably, in both these examples, the rotund Robert Morley appears to be the victim.
Long John Silver was sea-cook on two ships.
How about Jeffrey Dahmer? Would he count?
The cook’s assistant in The Hunt for Red October.
Samantha Caine/Charly Baltimore: Chefs do that!
Lunch Lady in Buffy the Vampire Slayer.
Typhoid Mary?
Paul and Marie Bland are serial killers in Eating Raoul. He’s a wine dealer and she’s a nurse, but they are amateur cooks who plan to open a restaurant (which they eventually do).
Gus Fring in Breaking Bad prides himself as an amateur chef, especially of Chilean seafood soup.
Paulie in Goodfellas and Clemenza in The Godfather give cooking tips while preparing spaghetti sauce for their fellow gangsters.
In Fried Green Tomatoes, the killer turns out to be Sipsey, the cook/
The chef who gave Homer Simpson a poisoned eclair?
Lisa: Dad, don’t eat that! It’s poisoned!
Homer: Eh, I’ve had a good life. (prepares to bite into it)
Lisa (desperately): Um, ah… It’s LOW FAT!
Homer: NOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO!!! (throws eclair. Explosion follows.)
Chief Wiggum: Boy, it’s lucky that thing landed in this smoking crater over here!
Awesome killer cooks. Keep 'em comming!
I thought of a somewhat obscure one: in one of the Sharpe episodes, there is a French cook who leads a group of murderous deserters (not “dessert-ers” :D). He’s shown killing a soldier at the start of the episode.
If Mrs. Lovett counts, then you would have to include Richard Boarst, the cook in The Cook, the Thief, His Wife, and Her Lover as an accessory to murder.
Maybe you want a subcategory of cooks who cook people, even if they didn’t murder them themselves?
On the Simpsons Treehouse of Horror V, Lunchlady Doris together with the other staff of Springfield Elementary kill and cook students to serve them in the cafeteria.
There’s the old Eating Raoul. Maybe not really a professional chef, IIRC, but close enough.