Q:

Ancelmo's laboratory - I have the key, but still need to kill the guards?

So I am on the quest to get the falmer decipher book from Ancelmos lab, and I've stolen his key so I can enter it, which is an optional task. But when I get in the lab there are bunch of guards I need to kill, or pay the bounty and leave. Of course If I kill guards, my bounty will skyrocket, and I can't really kill every guard in Markarth, to get away with it... so how do I get around this?

A:

Picking the lock, or stealing the key to the museum will make the guards hostile to you. From this point, you will need to go the sneaky way, or kill people. 
If you want to avoid this, you have other options: 

persuade Ancelmo (speech)
do something for him (Kill the spider or solve The Book of Love quest)

He will then give you the key, and guard won't be hostile.

