1. You assume she actually was his girlfriend. Thus far, I've seen no evidence to that effect.
2. It isn't cheating if you aren't actually in a relationship with the other person.
So he killed the two lovers out
You really don't know what you're talking about. The other initial victim was a Residence Hall Aide. They are upperclassmen students who live in dorms to help underclassmen transition into dormitory life. His relationship with the girl would have been strictly professional.