Cool topic, so its about funny moron characters becoming villains, I know a few,
Billy(Billy & Mandy) - He became a villain twice when he turns evil with freaky hair from Grim's haunted chest by having his scythe and he went totally bad by having Mandy's evil personality when Mandy became so cute/scared/innocent which Bad Billy scared me which he was worse than Mandy

:evil:.
Butters/Professor Chaos(South Park) - I always feel sorry for the poor kid which he tries so hard to do evil deeds & yet always fails, but his cool new big strong anime-like supervillain form was pretty threatening/dangerous though.
Peter Griffin(Family Guy) - If this counts, which he became a great villain by becoming a mean bully after beating up Chris's annoying bully & becoming a big shot corporate slimeball which Lex Luthor/David Xanatos would be so proud, hehehe.
Ron Stoppable/Zorpox the Conqueror(Kim Possible) - He rocks as an evil supervillain who's more diabolical/intelligent than Dr.Drakken

.
Beast Boy(Teen Titans) - In the episode,
The Beast Within, it was cool when he went evil by becoming a nasty jerkoff to a wild Man-Beast from that toxic waste splashed on him after fighting Adonis.
Barry(American Dad!) - He really scared me when he went evil/sadistic on poor Steve Smith without taking the pills which he was a great evilest villain in the show & Craig Ferguson did a good job with his dark evilness!!!!!!
Chowder(Chowder) - It was cool/hilarious when he became a wild Sleep-Eater beast whenever he sleeps to devour everything in Mung's kitchen & beating up on poor Mung/Schnitzel, hehehe.