I am not gonna spoil everything because its been a while since the update was out
We agree on Marie being influenced or another force influencing behind the scenes when everything happened.
Other than that, I never took the whole "Caleb being a monster" as him being a murderer or worse... From the beginning I always saw it as him being too tough on himself, the way he acts during the story doesn't strike me as someone capable of doing that, unless there have been more mind wiping shenanigans that haven't been explained, I think this is to show us that this was a traumatic experience for him as well.
To me, he sees himself as a monster because he has never accepted what happened and hasn't really moved on, yes his friend is presented as an awful person from his POV and because of that he probably deserved what happened but at the end of the day, Caleb sees himself as the murderer and at that point he was also just a socially awkward teenager, I can totally see why he would see himself as a monster after this specially when it cost him his relationship with Marie who probably just needed her BF to take care of her first and found him missing when she needed him the most.
That's the way I see it atleast, again we are missing a lot of information and I think this is on purpose, also Caleb is not the most reliable narrator till this point so we may be missing some context or even what we believed it happened isn't exactly what it actually happened, but again this is my interpretation who knows maybe in the end you are right.