There are two sites I would like to point out as well.
A map view of every mass killing (4 or more people) in the US since 2006. By the way 218 of the 290 listed here were done using a gun.
Also a list of homicides per country, (yes I know not every homicide is committed with a gun, but it is still pretty telling). I suggest sorting it via Rate which is homicides per 100,00 people.
Both of these sites tell me that the US has a problem with gun violence. Now as to whether the solution is to get better mental health treatment, limit gun sales, limit types of guns that are sold, or anything else, I am not sure. But like most problems today there is not a simple solution, and anyone who says that this will be solved with one thing (whether it is more guns or no guns) doesn't understand reality.
But in the US we can't continue to ignore this, because it is going to keep happening until we do something about it. I just wish we had a government that would attempt to do something about this.