Well, it literally starts with him telling us he's a monster. And yes, he literally is a monster, but we don't know that at the start and neither does he. So, dreams are a big thing in a narrative because they tell us things and truths about what's going on even when the characters aren't aware of these things. Andrew's dream tells us what he doesn't want to admit to himself, that he's a monster because of the things he's done, specifically, that he's the monster that caused Julia's death.
Then, he runs away from his own guilt, even though he tells himself he didn't do anything wrong. Well, why would he feel guilty then? Everything his girlfriend says when he hallucinates her is coming from his own subconscious. Yes, he tells himself he's not at fault, but that's just in line with a pattern of him not taking responsibility for his actions and very much what an abuser tells themselves. They don't want to admit they are the monster, so they put the blame on the victim.
Even before we see the flashback, we see Andrew blaming his dead girlfriend for his situation. So, he's blaming a dead woman for him choosing to run away from his own guilt.
The flashback is a giant red flag from the start. We see him following Julia, then everyone around him tells him she doesn't want him there, something we then find out is true when we hear Julia tell him over the phone. she mentions her friend advising him to separate from him, and says she's looking back at past actions of his she's excused. So that tells us he's done things that have raised alarms for people around Julia. Then we find out how Julia died, and even though he blames her, it was his fault. She wouldn't have been there if not for him not respecting her wishes to not see him and dragging her out of the party. He doesn't even want to take responsibility for the crash or get medical help because he doesn't want to get in trouble.
One more thing I noticed is the post Julia's friend made. She mentioned she was grieving Julia along with their friends, because grieving someone means they are loved. Well, Andrew isn't really grieving. He's haunted by Julia and his guilt, but he doesn't really grieve her. He's actually trying to run from her memory, so that tells us more about his thoughts. Again, he's putting himself first.
Deep down, Andrew feels guilt, but his actions show us he doesn't want to bear the consequences of his actions, he wants to forget about Julia and the things he did. But, because the things he did ended up with her dead, he's now forced to see the truth of who he is.
Sorry this got longer than expected, but it's an interesting story. The signs are there, you just have to pay attention and think about it. Don't take the things you're told at face value, because sometimes you're dealing with an unreliable narrator like Andrew.