I'm doing the thieves guild quests and I was sent to get a falmer translations book from Calcelmo in Markarth. I killed the spider and got the key from Calcelmo to enter his Dwemer museum. Here everything is fine so far.
Then I enter Calcelmos laboratory from within the Dwemer museum. Over there are Wizard Guards that are hostile and attack me. It is also strange that Lydia, who is supposed to help me, keeps telling me that I should not be there and refuses to obey to my commands.
Since I got this problem already previously and went back to an older save game I wanted to double-check, so I killed one, two three Wizard Guards and then backed out out of the lab and returned to the museum. The Guards there were still friendly.
I then walked through the lab, killing the Wizard guards on my way. I did NOT get any bounty message while doing that. I copied the falmer translation stuff from the stone and returned through the lab to the museum.
Now, after I left the laboratory and re-entered the museum, the Markarth guards are hostile and attack me. Even when I put away my weapons they keep attacking me - I don't have a chance to talk to them anymore.
So what's wrong here? How can I finish that quest without making the Markarth guards hostile?