I believe many of the mass shooters are deeply and darkly affected by drugs now laced with a drug that causes depression, anxiety, and paranoia. I think it's called Fentanyl if I remember correctly. It's been awhile since I read about it.
The article was talking about how drug dealers are approaching young people (and sometimes older) as though they want to be their friend, offering them "Free drugs" that are highly addictive and laced with this terrible Fentanyl drug. Then charge them when they are addicted.
Anyway, the combination of depression, anxiety, and paranoia...not to mention the dark stuff they watch on their computers, political hatred, sensationalism in the media, etc. is what's causing them to execute these horrific plans of mass murders.
Regarding Aunt Marg's question...the 2nd amendment has little to do with it other than the guns that have those rapid fire multiple rounds of bullets...those should be banned (although not all murderers use that). These murderers will find a way no matter what the regulations. Most know how to make their own bombs from the internet.