The US experiences gun-related murders, school shootings and other violence quite frequently. It obviously is a direct consequence of the mass-ownership of guns by the general public. Simple mathematics can explain the regular occurrence of these shootings in America quite well.
Since it's certain that at least some people among the entire population of America are going to be mentally unstable, and since it's also sure that at least some people among the overall population are going to have some sort of vengeance with some other people [for whatever reason], and so on, plus if all or most of America's population owns a gun/pistol/rifle, then it becomes quite natural that at least a fraction of these angry/disturbed/enraged/violent people are going to pick up their guns and actually use them on others. It's as simple as that.