What changed America? I doubt government engaging in gun control is going to solve the changes in America that have created mass shooters.
Are you looking for an honest answer? If so, the honest answer is multifaceted and complex. I think if you've been around awhile, you can identify the changes in society that have lleadead to this.
1.) The Gun Control Act of 1968 was a knee jerk reaction to the RFK and MKL shootings, as well as the previous JFK shooting.
2.) The Baby Boomer Generation - also known as the "Me" generation-took to the streets and changed Pop Culture. The area that changed much for this discussion is the questioning of all authority and the concept of "speak truth to power"! With that, we began to get a break down in societal and family discipline. There was now absolute right and wrong and the idea that the rules apply to "you" but not to "me".
3.) Breakdown of the family unit. Divorce, unwed mothers lead to an explosion of homes with no father figure. No father figure makes everything more difficult, especially family discipline.
4.) The concept that has resulted from all of the above is there is no "Do Not Cross" threshold that is hard and fast. No absolutes in terms of crimes (misdeeds) and punishment. In the old days, if you back talked a teacher, she'd paddle your bottom or the Assistant Principal would do the same, and then the Old Man would tear into your ass when you got home. Usually, that solved that issue for an extended period. Now, a teacher can't discipline a student and corporal punishment is darn near a felony.
5.) Boys no longer are allowed to settle things Mano a Mano. Yep, especially right up until they are Juniors and Seniors in high school, many things in the bygone era were settled with a fist fight on the playground. Bullies were backed down., disputes put to bed. Many times the combatants were once again fast friends 30 minutes post action. It cleared the bad blood. Even enemies got it out of their system most of the time. It's called conflict resolution.
6.) Today, instead of a schoolyard brawl, kids take to Social Media to bully somebody. They can do it in the safety of their own bedroom, be as nasty as possible,get their minions to pile on, and expect zero repercussions. they can write things in Social Media that would get their little asses whupped but good in person. Now they are fearless with lies, innuendo, and all manner of liable and slanderous activity that can be broadcast to the entire planet. Character assignation at its finest.
7.) Columbine. Once Eric Harris and Dylan Klebold [pulled the Columbine HS massacre, then the Genie was out of the bottle. Prior to that, no kid would think to go to school to kill people. After Columbine, schools were no recognized by sickos as targets of opportunity.
8.) Everybody is "special". That bullcrap has been foisted upon our kids by the Leftist AND by the single moms that their little darlings are "special" and noteworthy. In the bygone era, your had to achieve something noteworthy to be considered "special". you had to be the Valedictorian, the QB on the football team. you needed to bash the winning home run or score the touchdown to earn the accolades. You wanted to be successful in your career to be seen as "special". Otherwise, you'd be just another schmuck. Mommy now tells them all they are "special," even if they are a "C" student, can't walk and chew gum, and play 8 hours of video games each day.
Lumping all of this together, we have self absorbed kids-especially boys-that have been brought up in a culture that lacks discipline, lacks absolute right and wrong, They have not been forced to learn the social skills of to deal with success and disappointment that is life. They have no boundaries for behavior. When I was in high school, we hunted before and after school, kept the guns in the trunks of our cars, and NEVER considered taking them out because we knew their would be BIG trouble. That was in a time when skipping school at lunch to go to McDonald's was the height of naughtiness, and is you got caught you had a week of detention or 5 swats on the ass with a paddle (ask me how I know!). If the "NoGo" line was skipping school to go to McDonalds, then the concept on bringing a gun and killing everybody insight was not even fathomable. Now, the line is faint and moveable. No absolutes. No capacity for problem resolution. No father figure to teach you to be a man. No way to settle disputes other than cyber attacks. No respect for authority.
Look at how our duly elected POTUS is treated by the MSM, Democrat Party, Hollywood, and the Press. No repect for him or his office. Obama sucked as much or more than Jimmy Carter, but the office was respected. No more. Left doesn't like the outcome of the 2016 election, so the rules don't have to apply to them. They will just nullify the result and drive Trump from office. See, this is the message kids are living and learning.
I'm sure other can add, but that will give you a good start on the problem.