Kill servants within the room to obtain a "Black knight workshop key". If you did not get a key, use a tool dropped by the servants to search the benches until you find the key.

 

The tool being referenced is not dropped by the servants. There are 4 workbenches in the room. One of them (the workbenches, not the servants) has a tool on it, which you should use with the other three benches to get various metal parts. The tool will break after each bench, so you must get another one each time. You should get two metal strips, and a metal hook. Use the parts on each other to make a gaff, and use this on the shelf on the north wall of the room that visibly holds the key. If you attempt to get this key without the gaff it will say you are unable to do so with your bare hands.

 

Thus you are able to proceed without murdering any of the servants. They will not interfere at all during this either.

Apologies for misunderstanding your initial submission - I've now fixed this up and removed the part about murdering servants.

 

DK