Personally, I suspect this man had serious mental health issues. He was

at least a verbal abuser of his girl friend (maybe a physical abuser behind closed doors) This country has a terrible record when it comes to caring for the mentally ill. His brother was shocked, but said he's had little contact with the shooter for 10 years, so he wouldn't have known about subtle personality and mental deterioration and some are quite good at pretending normalcy for short periods.
As for guns, blaming them makes about as much sense as blaming the auto that kills someone in the course of an accident. Do we ban all cars? No one needs an automatic rifle for hunting - or anything else. but rifles and pistols can be used for a variety of legal activities. I am a gun owner and have been for many years. The only thing I have ever "killed" were targets. I don't hunt - don't even like the idea of hurting a fellow creature. Unfortunately we do have a past history laced with violence and there has been an increased acceptance of violence in recent months as a means of dealing with a variety of issues