There used to be a thing called the “Social Contract,” unwritten rules about behavior and things you just did not do.
No one robbed trains, till the Reno bros did it, then train robberies became common.
No one robbed a bank in daylight till the James brothers did it. Now bank robberies are still common.
You did not shoot up churches schools or places where people gathered, till someone did it. Now it is common.
Of course, closing down the mental institutions in the 1970s and letting the crazies back out on the street certainly did not help.
About every mass shooter has shown clear signs of mental problems but the common excuse is now “He is just off his medicine” still is the main reason to keep them among society and killing.
Then there are violent video games and revenge movies galore made worse since 1969 and the removal of the Hays Code in movies.
Social Contracts like religious traditions are passed on from father to son.
There fewer and fewer households headed by fathers as time marches on.
Fathers socialize sons and teach their sons to live peacefully in the society in which they are rauised.
Sons that are not socialized have no root in the society and become alienated from that society and grow to hate the society.
These alienated boys (or girls that believe that they are boys) find reinforcement of that hate on the internet and also find the techniques and tools to attack that society.
Gun Free Zons make mass murder laughably easy.