The only legal way around that (and you were asking about legality) is to pay a LOT of money in licencing fees. It doesn't matter if it's a free game or not. It also doesn't matter whether or not it's for personal use, though so long as you didn't distribute the game, the chances of getting in trouble over it are next to none.
Looking at reasons behind mass shootings is a bit of a red herring. While these incidents are indeed terrible, the number of deaths caused by mass shootings is only a small percentage of firearm related homicides.