Unwanted Guest

Trapped in a psychiatric hospital, Joan witnessed nine friends slowly diminish to only four remaining targets in the building. All four friends gather to find the killer.
“One of us must be the murderer.”
Joan questions Mason’s alibi and is still unsure of the killer’s identity.
“Who are you talking to? Who’s Mason?” The other two question.
Joan watches Mason suddenly open fire and murder the last two friends trapped in the room. Joan takes her medication to make Mason disappear again. She opens her eyes to find herself alone with the gun in her trembling hand, covered in blood.