I have thought about it, and contacting the police straight away would only be the right thing to do if there was some imminent danger. I probably wouldn't have mentioned it, except that it was the sort of thing which can pose an indirect threat or lead to behaviour which does hurt other people.

Anyway, it appears I got the wrong impression anyway, and he was only obsessing over the hypothetical possibility of doing things, rather than actually doing them. So this is one of the times when its good that I didn't impulsivly do the first thing that popped into my head and instead stopped to think about it.