I am hesitating to write this because I don't want people to take it in a direction it isn't meant to, but as a teacher in a high school, I can almost guarantee the shooter's teachers over the years has had misgivings and have tried to say something but weren't taken seriously. A lot of times we can see little things that make us think twice and raise red flags and when we try to speak up, we are dismissed, for a variety of reasons, usually by those who have more power than we do and by parents who either don't want to hear that their precious angel isn't perfect or are terrified of bigger issues.
Obviously I do not know that this is in the case in this situation but I am seeing more and more info coming out that this person was likely very mentally ill but there was no formal diagnosis and people questioning why wasn't his issues addressed before this. By the time students get to us in high school (especially as upper classmen that I teach and interact with), their personality, moral compass, and thought process is pretty well fixed (not saying that people can't change but you can see where some of these kids are going to succeed, some are going to have major struggles in life, and some just plain creep you out because something is very wrong). We try to get the kids who raise these red flags with us as much help as possible but often times we hit major road blocks either from admin, parents, legal restrictions/litigation and it scares us because we see what the future could possibly be but no one will listen to us.
All this long rambling to say, I wish this event and others like it, had never happened. It terrifies me that we as a society have not found a way to keep students (and their teachers) safe from acts like this. I wish that there was more emphasis on helping people with mental health problems, and (a more controversial opinion, I know), I wish there was a way to require mental health treatment for those who need it. In the end, I just try to do the best I possibly can for my students and I hope they turn out to be good people who contribute to society positively.