Observations
First off, hang in here, Nichols250. It sounds like you've got a good head on your shoulders.
Second, hindsight is 20/20. Monday morning quarter backing and finger pointing seems to be a hobby on forums and chat rooms.
Third, this is America. You can't arrest and remove somebody simply because they are weird, scary or reclusive. If that was the case, many high schools and the NYC subway would be emptied on a daily basis.
That's a matter of free speech and freedom of expression. Can you imagine the ACLU and other agencies reactions had they tried?
They did suggest counseling, but there wasn't much more they could do unless he had actually made threats of caused damage or harm. "The Minoritory Report" was fiction and, if memory serves me well, quite a fine allegory on why that line of thinking is dangerous.
Finally, Columbine was one school which could be easily locked down. This was a huge campus and essentially a city onto itself. He could run and hide anywhere and apparently did. IMNSHO, some armed security might be a good thing in a facility that size.