How to get past the guard and servant undetected

This is actually the trickiest problem I've faced so far, playing the new Thief.

There's a ventilation shaft in the room next to the safe room, which leads back to the entrance with the stairs. You'll need to go through that shaft and exit it¹. Stick close to the shaft entrance, or you'll be spotted by the guard.

Shoot a blunt arrow on the switch, by the staircase leading downstairs; this will turn off the lights and give you a good opportunity to sneak upstairs undetected.

You will obviously raise the guard's suspicion, but at least you won't be detected.

¹: You might want to save beforehand.

When you first enter the area with the freaks, you will see an animation with a freak running away from you. This freak will disappear in a hole in the wall. If you are quick enough, you can shoot it with a blast arrow and get an additional kill. AFAIK this is the only way to achieve the 12 kills/knockouts in this chapter.