i think he definitely was going to kill her regardless that night, but it was probably originally planned to be more of a romeo and juliet star-crossed-lovers type of tragedy. he didn't know she was going to invite alex. and he definitely didn't know she was going to ask him to buy the dudes ticket for him, which is just so ballsy on her part given she knew that brandon was obsessed with her romantically.

throughout the night as he saw them holding hands and chatting, and eventually found them making out against his car - it switched from being mainly focused on the suicide part of the murder-suicide and more on the murder part. it became fueled by rage, which changed the nature of the crime itself.

i mean, waking her up just so he could initiate an argument and berate her. "did you like kissing him?" "you can't go one day without doing something slutty, can you?", slapping her across the face, etc. it became less of him wanting to die together, and turned more into him wanting to murder her in a very painful, up close and personal way.

he easily could have killed her in her sleep, but he chose to wake her up so she had to face him. he grabbed her by her hair and yanked her head back so he could cut into her throat. for kim and all these documentaries to say the kiss with alex made no difference because he was going to kill her anyways is stupid imo. it made him 10x more angry and taking pictures of her afterwards was just his final "fuck you"