I can keep going. Are you still ready to dance to this tune?
The murderer killed Simon inside the locked room. He then unlocked the door from the inside and left the room. A different person then entered the room and locked it from the inside. That person then died of poison, heart attack, illness, accident, suicide, asphyxiation, trap, time activated device, remote activated device, motion triggered device, or any similar means that doesn't require a murderer in the same room!
If not, they are hiding in an area only accessible from the room but not actually part of the room!