3 months is not a long time to know someone.They probably did like each other,however as Nel pointed out most of the "loving messages were quite short" which is typical of a relationship in this stage.I believe she was going to leave him and he simply lost it! He absolutely must have know who was behind the bathroom door,its improbable to think otherwise.
+jamc666 I agree that B,C,D, locks him down for murder. I expect Nel to ask Oscar about that pause before putting 3 more shots through the door. The pause after the 1st shot is the premeditation. The final 3 shots and moving the gun to the right is the intent to kill.