He has a very high IQ that has a problem with ever being met with discipline. This will end up in mass frustration that leads to hormones accepting his dates. He kills in response. The man kills those that deny his sexual demands.
This was mainly about robbery. Thus far, there’s no evidence of sexual assault, although she was wearing a shirt and just underwear when found. There may be more learned about that in the future, but as it stands right now, sex doesn’t appear to be the motive.