You should check out the Mirage or Image Turtle comics, he's insane... literally. In the movies and the cartoon serieseses they always presented him as some justice-oriented lovable goof who was mostly there for comedic relief and tended to get in the way as much as he helped out, but in the source material he was very much a violent and borderline-psychopathic vigilante who would fly into fits of berserker rage whenever he was engaged in combat. It's pretty sweet. In fact, he was one of the only people, other Turtles included, that could hang with Raph whenever **** got really real.
Goongala! Goongala!