Why not let them due by the method they murdered their victims?
The nitrogen is a as close to a perfect murder that is possible. Seal up the bottoms of any doors, stick a hose under the door and open the valve to a big nitrogen bkttle. The nitrogen stays on the floor and rises as it is heavier than air. Eventually the nitrogen gets above the victim's head who is hopefully asleep. The victim is without oxygen and dies. Remove the tube piping nitrogen into the room and unreal the doors. When the room airs out, there is no proof is left behind to why the victim died.