That's why I like this game: you can have nuanced conversations about characters like Killian who are not two-dimensional collections of tropes. He's a terrible son, a worse boyfriend, and even though he realizes he's one he'll do nothing to change himself. But at the same time he's the best friend Edwin could wish for. Not the best influence, certainly, but if push comes to shove and Edwin needs his help, Ian'll say "I'm in" before hearing what the actual problem is. Loyalty like this is rare, which is why Edwin is so apprehensive about his relationship with Mina.
Why is Ian so self-destructive, though? My guess is because he's afraid of failure. Life at its worst is swell for Killian, it's not like he has to strive to survive. He's got a swanky apartment, a well-paid job that is not that strenuous, enough cash and charisma to pump and dump a new girl every night out. His mom might consider him a loser, but how many losers can live a life like that? Yes, it feels empty sometimes, but he can fix that by drinking another shot and not answering his mother's calls.
Mina breaking up with him reminds him of this. She's a living proof he's a bad boyfriend. It's one thing to pick a girl up, let her cook you breakfast in the morning and then ghost her. You're a dick, but you planned to be a dick. But this time he wasn't horrible enough for that. He tried to be a boyfriend, and it turns out he's not good enough. Why get a degree and a serious job if your employer will fire you just like Mina dumped you? Just more fuel for those considering you a failure.