That's the way mental disorders work guys.
I'll break it down for you:
1) He witnesses the batman shooter and all the news about him afterwards, he has similar murderous fantasies.
2) He watches batman and he sees "sandy hook" on the map, and his brain plays tricks on him and he sees it as a sign or message that he should shoot up some place too.
3) he shoots up a school.
Clear case of mental disorder, not some conspiracy.