Even if he really is the killer or invlolved, I really don't think he acted alone.

I don't think this kid just thought to himself "I'm gonna kill Charlie Kirk"

Then just drive down there, mounted the building without suspicion, shot him then just left all by himself and then managed to escape to Washington UT on his own, then happen to admit to his dad and his dad happened to know where he'd be and tell them where he was.

I don't think this is what happened.