Perhaps something happens in the "past", that makes Marcus aware that he's somehow being deceived, and he doesn't wind up killing anybody.
Part of the problem I have with the alternate universe was the seemingly arbitrary decision that the split occurs at Kirk's birth in 2233, instead of much earlier. Marcus was alive before then, so what was he like in the "prime" universe? What happened after Nero's incursion that caused Marcus to become what he was?
I've not seen Into Darkness so can't comment specifically on Marcus' personality.