This might be a hot take but I'm 100% certain he did not kill her himself at the very least. He seems to have been on tour which gives him a good alibi.

There's a good chance that either he had someone kill her without getting the details of how she was killed (otherwise he would have had the mess in the Tesla taken care of), or they broke up and Celeste got herself in trouble with the people she was (allegedly) being pimped out to.