Bill
Mental health issues play a part , no doubt.
As somebody who was an administrator in a leading mental health and research facility for a few years , I don't recall any statistics showing that the percentage of the population with mental health problems is significantly greater in the USA, versus Canada, Japan, Australia , UK, etc, and while no nation us immune to gun deaths, the percentage of deaths ( adjusted for population size differences) by guns is way higher in the US. The percentage of guns owned per person in the US is much higher in than it is in other countries, so too many guns and in places ineffective gun policy also plays a part.
Serious question here , this fellow reportedly had 15-20 semi/automatic weapons and these are the weapon of choice in most of these "mass" killings. Are these types of weapons that are used and designed for military use and brutal , quick killing of many people needed by your average citizen gun owner? Do you think eliminating them would help reduce these tragedies ?