just because someone has a lot of guns means nothing to their mental state. someone could own 200 so called "Assault rifles" and never shoot more than paper.
just as someone who owns no legal firearms couldn't go out and buy one off the street and kill everyone in a McDonald's.
The Number and type of firearms someone owns really is a moot point. the gunman from sandy hook (i refuse to give his name more air time) didn't own any guns himself. he blew his mothers face off getting her guns. she allowed him to take her guns to the range. but was also trying to get him mental help and kept getting denied. she really shouldn't have let him anywhere near them, but he would have gotten the guns he needed no matter what. it was just easier to kill his mother to get hers than to go and try and buy one on the black market. he could have just as easily gone to "visit" one of the people from the gun range and killed them for their guns.
the main issue here is mental health and gangs. if we could separate the numbers into gang related, suicide, legal gun owners, and police shootings. Im pretty sure suicide and gang crime firearm deaths would be far higher than legal gun owners.