Ok. Let me break it down in baby steps. After reading some reviews I got pretty confused myself.
Troy was drunk.
Troy and Gabriella had a fight.
At the end of the story, Troy kills himself.
We can assume that Troy had killed Gabriella.
you know how sometimes when you have a hang over, your memory isn't that great. and then after a couple of days or so, it will come back? Troy didn't kill himself until after he remembers that he killed Gabi remember his promise he made to himself?.
I'm working on another few chapters that explains what happened at the party and the eventual death of Gabriella.