He has to be planning this for weeks, if not months, so this is not a flip out.
I am thinking there might be a bigger political picture (beyond gun control) that I can’t see.
Or He might be a Left Wing Breivik.
Or he might have been the fall guy for someone else.
I'll agree with the first part. He probably planned this for a long time, enjoying the fantasy. I cannot imagine that, but the typical criminal who hurts people puts a lot of time into the anticipation. I chose my words poorly with "flipped out", but whatever mental problems he had to begin with presumably became worse over time.