Hey P, everyone will be coming up with theories as to why this occurred. Really, there is no "one" answer, outside of severe mental illness.
For example John Hinckley Jr, obsessed with Jody Foster, and tying to get her attention, ( I wonder how he would have reacted if he would have found out she was LGBTQ).
Mark David Chapman, felt betrayed by J. Lennon,
Eric Harris and Dylan Klebold,, Wikipedia "In 1996, Eric Harris created a private website on America Online, initially to host gaming levels he created for use in the video game Doom. On the site, Harris began a blog, which included jokes and short journal entries with thoughts on parents, school, and friends. By the end of the year, the site contained instructions on how to cause mischief, as well as instructions on how to make explosives, and blogs in which he described the trouble he and Klebold were causing. Beginning in early 1997, the blog postings began to show the first signs of Harris's ever-growing anger against society." Both of these kids were doomed from the beginning.
Adam Lanza, diagnosed as having some degree of Autism, later revised by the "experts" to, "Oh by the way, yeah he was a psychopath.
And finally Nikolas Cruz, was killing small animals for fun, not because they are vermin infesting the house or yard, frogs generally are not considered vermin, but just for fun. This is generally an early indication of a Psychopath.
Having an adult conversation about Mental illness in the US is taboo, and when it comes to second amendment rights it gets even more complex. For example, should a person who at some point, has taken or is taking antidepressants be denied the right to own a firearm? If so how would the Gov't know, would one be forced to release their medical records to the Gov't before a permit was issued? Very tricky stuff here.
Really I don't think these killings have much to do with an "Attention" problem, but more a core Psychiatric issue.
The Ever Adult VIPER