Here is a picture of the room from the viewpoint of the doorway, to start the execution I teleport somebody to the location of the torch:
After teleporting somebody in I quickly run to the doorway and block it up so they cannot escape, like so:
Then, directly to my right is a staircase which leads to above the chamber, I run up there quickly to administer death!
Once up there I take two TNT or two lava buckets and insert them like so. The lava will automatically flow down and fill the chamber and when activated the TNT will fall into the chamber and explode!
Here is a picture of what the chamber looks like after being flooded with lava, I removed the lava from the forward hole so you could actually see inside, nobody is surviving that!
The chamber only has two weaknesses that I have seen. Firstly on my server only admins can teleport people to them but anyone can teleport themselves to another player meaning people can just tele out if they think fast enough. I usually demote them to guest rank before killing ;). Also, a player can block up the two holes to halt the TNT or lava; I have actually had the walls of the upper room blown to bits before when someone blocked up the holes preventing the TNT from falling. This can be fixed with a demote as well.
This is a really fun and interesting way to kill people. Of course if it doesn't work and you have a score to settle you can always do this....