5. Seemingly Letting Michael Die
For all his loyalty to Michael throughout the vast majority of the game (even through refusing to help Franklin kill him if you choose Ending B), upon finding out the truth behind his 10 year absence, he happily lets the Triads who are tracking him down take Michael instead. Although Michael and Trevor's relationship often mimics that of a rowdy, unstable pair of lovers, it's Franklin who happens across Michael's predicament through lack of contact and manages to rescue him from a slaughterhouse just as the Triads were sending him down to be turned into mulch. Many have argued that Trevor was well-aware of Michael's skills, and therefore this was just (in their world of crimes and debauchery) a way of getting him back for killing their partner Brad and faking his own death. I personally think that's a bit of a reach, as I liken Trevor's rationale more to that of the Joker's explanation of his motivations to Harvey Dent in The Dark Knight; he just "does things", fuelled by whatever thought is at the front of his twisted psyche at the time, occasionally falling back on rulesets and empathetical ideals when the writers felt like it.