What's the conspiracy theory of choice here?
I don't think the ISIS claim is remotely true.
Based on the official police/FBI/media reports - If I had to make an order of my most likely theories despite the fact I consider all these theories unlikely to extremely unlikely-
1- He was a leftist/antifa so "they" are trying to hide his motives until the story blows over.
2. He was secretly insane
3. He's a deep state operative working for ? ? ?