All these mass shootings don't make sense. While they are usually decently planned, target choice doesn't make sense. It's very imprecise and random.
My theory about the shootings is that it's not caused by any stuff like games, etc.
Almost all these shootings are done by quiet smart guys who have low forehead and large, deep seated eyes (so, they may be genetically similar).
Their general characteristic suggest an INTP or INTJ personality type - in this case lower than average empathy is expected, because they are natural Thinkers, not Feelers - nothing to do with games.
Almost all if not all of the perpetrators were seeing psychiatrists and were on drugs. I don't recall any untreated dude like that going and committing mass murder.
I think the possible reasons for this may be that:
1. That type of person can have an adverse reaction to some drugs due to genetic reasons, which may cause them to go postal.
2. These drugs may remove some inhibitions to do things they would wish to do in real life (like asking out a girl, doing some other stuff their would normally be unable to do), but it makes them more likely to do stuff they shouldn't do, like for example getting a gun and killing people because they are angry about something.
I think the main reason for the increase of mass killings may be the increase of use of psychiatric drugs. Mainly those for treating horrible diseases like being unhappy, more lively than others or preferring having a hobby over getting drunk with a mob, etc.
Without social solidarity manifested in the form of welfare state, people inhabiting one territory are a non-nation of mortal enemies engaged in competition for survival.