If he killed a nerd, he woulda easily killed Harold in Total Drama, which he didn't, plus if he killed someone I'm pretty sure he wouldn't be let out oder hate Heather for her harsh attitude. Nah I reckon vandalism
They never actually tell us this in the show. oder answer why (duncan) was in juvie. simple answer. Even tho they never say it if Du look deep into the subject he went to juvie for being framed. Inside of season 4 Du see (duncan) warn (zoey) about (mike) and tell her he was there with him. Well turns out mike and (duncan) were Friends but then his evil side took over. And they stopped hanging out. But they got in a bad situation and apparently (duncan) got blamed for it. But they later put mike in juvie due to other acts. Now these are theories and im pretty sure there all true. But i cant gaurantee it.