So, I was Idolized by the NCR. I had been to Michael Angelo's earlier, but hadn't bothered to trigger his quest. I decided about 20 hours later to go and finally do it, and while I'm walking around his shop, an NCR MP comes out of nowhere and says something about "nobody ****s with the NCR!" and attacks me. I kill him and two assistants of Michael Angelo who turned hostile for some reason, now I'm a Dark Hero. I suppose it's possible that it had something to do with me previously (about 30 minutes before visiting him) doing the Helios One mission and routing the power to Archimedes II, but I didn't kill the NCR (even though I did set it to kill them all and then set it back).

A few people, myself included, have noticed this happening inside the NCR compound after talking to Amabassador Crocker, this is the first I have seen of it in Michael Angelo's shop though, I had been in there previous to the unwarranted harrassment so I wonder if that location is a trigger for that particular bug.

Happened to me too. I reloaded the autosave and went back outside and interacted with the NCR character near the door and then went back in to complete the quest.

I was able to complete the quest regardless, I just now have Dark Hero status with the NCR (which is fine as long as they aren't shooting at me and they'll still talk to me).