I have been mercelessly slaughtering everyone. I have killed most of the people on the strip and the securitrons, benny, everyone in his casino, and mister house. shortly after I killed the president of the NCR and then killed caeser I fast travel back to new vegas and I'm asked to submit to a credit check. Then I walk into the tops to go see yes man and there are floor managers again, plus the cashier is alive again which there is only one. Haven't looked around much to see where else there are zombie NPCs but I just thought this was a very unusual bug. I'll probably just slaughter them all again in a blood bath.

I'm searching very hard for a question in there. You had to pass a credit check to do the first slaughter, so the second one is puzzling and might have something to do with the resurrected NPCs, but you seem to have found a work-around. If I missed anything, let us know.