The very first issue ends with the turtles searching out and hunting down Shredder (no long and storied rivalry for the turtles to build up animosity toward him or anything; they were total strangers up until the point the turtles jumped out of the shadows and tried to murder him). The encounter ends with a brief fight on top of a building, where this happens:
Turtle Power, motherfucker!
That's Leonardo -- the boring moral center of the group, the generic good guy, the default leader character that nobody wanted to pick when it came time to declare which turtle you wanted to be -- and he is straight up brutally murdering a man with a sword. After Leo stabs Shredder, the rest of the turtles surround the mortally wounded man and tell him in no uncertain terms that he can either be dishonorably murdered (by them) or else honorably commit seppuku, which is essentially suicide by disembowelment.
He refuses and eventually dies while trying to kill the turtles, but really picture that first scene: Wisecracking Raphael, nerdy Donatello, noble Leonardo and Michelangelo -- fucking Michelangelo with his surfer accent and cowabunga attitude -- are all standing around an injured man trying to force him to cut his own guts out. And this was not a weird, unique misstep in an otherwise harmless comic. Nearly all of the early Turtles books were absolutely filled with the kind of ultraviolence that would make Alex DeLarge dry-heave stomach bile onto his loafers.