One thing does not equal the other. Failing to stop it, even when they had some information that should have tipped them off, does not equate to staging a mass killing.
That's just nuts and people should be ashamed to spout such nonsense.
There is usually nothing the authorities can do to stop these things. They are inherently unknown and unpredictable until they happen. That's just reality. In this case maybe they should have been more alert, but that's just human failure. None of that points to dark forces being in secret control. That is irrational paranoia and there are pills that can help.
There is a simple procedure for mentally ill people threatening violence. You “Baker-ize” them, which means you commit them to a mental hospital for 24 hours, or longer if he is determined to be a threat to himself or others, and is not stabilizing. What crap that nobody could do anything.