You can't kill those guards, look for another solution.

[spoiler]



When the glass of the holding cell is shot, run to the closest corner. There's a door there.
Run down the stairs and straight forward into the room there.
Pick up the knife and kill the person in the room. There's health there.
Walk back towards the stair and stay just around the corner. Wait till you here the door go open and close again. That means guards at the holding cells concluded all prisoners are dead. Now go back to the room with the holding cells (you might actually see the guards leaving the room). You'll need your knife to leave that room as you can't leave through either door.

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The problem is that the guards aren't gone. They're all bunched up in the exit to the stairs. But like I said I got past it with cheats and random luck.

Never had guards at those stairs.

You go to the closest corner of the room. There's a door there. Once opened, you'll have to go down the stairs. At the bottom of the stairs turn left. There's a small infarmery in front of you. Go through that door, pick up the knife and kill the person there. There's some free health in that room. Go back to the stairs and wait just around the corner. Wait a while and the door opens and than closes again. That's your cue that everything is save.
Go back to the room with the holding cells. You might actually see the guards leaving the room. Walk towards the door they brought you in through and look at your left. There's a hatch there with some planks on top of it. Remove the planks with the knife and operate the hatch. You fall in some kind of garbage disposal place.