We get similar atrocities in Europe, but the weapons used are usually knives, machetes etc.
Except the example you have provided is like comparing apples to oranges. The children in Texas had to be identified using DNA they were so mutilated with bullets.
There is one difference between Europe and the USA and that is our insane gun culture that was created and is now supported only by the right extremists. Our courts are even conflicted about whether or not the 2nd amendment even includes the right to bear arms.
Reagan shut down all mental health institutions while he was in office back in the 80s. Even if the family members of the suspect shooter(s) would have tuned them in to authorities before these mass shootings, there are no laws that support holding anyone for medical treatment of any kind against their will.
I can't in all honesty relate to American gun culture or voice a fully informed opinion on the subject, all I can do is offer my 2 cents as an outsider.
Guns are basically unheard of here and are only owned by farmers, the police and a tiny percentage of career criminals. Going to a store to purchase a gun and ammo is about as alien as it gets to us Brits.
I won't deny that there does seem to be a problem with gun violence in the US, but many Americans have been brought up to believe that gun ownership is their god given right.
The problem with stripping law abiding citizens of their guns is that it would result in just criminals and the authorities having them, and who would want that?
Not a clue what the solution is, but it's safe to say that the state is very keen to disarm most of the public. Make of that what you will....