I don't think we know that he pushed her to the ground or even that they got into an argument. That was apparently just one version of the story he told. It doesn't make much sense that she would then willingly go into the woods with him. It's more likely he lured her there.
From what we have heard, the only motive in this case is that he wanted to kill. He didn't need to be angry with her. We know he made statements to friends about killing someone for at least a month, but he may have been having homicidal thoughts for months or years. He may have had fantasies or urges for a long time.
This case reminds me of 14 year old Claire Miller, who stabbed her sister several times as she lay in her bed. She was allegedly having suicidal and homicidal thoughts before the murder. I think she chose her sister because she was an easy target. But she appeared to be remorseful after the murder.
Aiden seemed boastful and proud.
I believe if it hadn't been Tristyn he was with that night, it could have been anybody he chose to kill. But she was with him, and it was the perfect opportunity. Once he had her alone in the woods, she was vulnerable and an easy target. The head wound/wounds could have been a result of any number of things.
We may never learn all the details of what happened that night, but we do know that he stabbed her at least 114 times and she fought for her life.
I have no doubt that if he hadn't been caught he would have killed again. He killed her for the thrill, with no remorse, and was proud of what he'd done. He didn't need a reason. He didn't need to be angry. He had the makings of a serial killer.
IMO