When a loving man lets his wife be detained in a mental hospital because she keeps seeing their dead son, he starts seeing the boy himself. After a long mental struggle he finally accepts his wife wasn’t mentally ill after all and he lets her back in only to find out she isn’t his wife anymore, but possessed by a demon which already took their son in the past. If he really loves her, he has to kill her. (just as she did with their son in the past)

Thanks for ruining the ending... :)

Thank you for the effort. It 's coming closer.
It 's not written yet. Working on the very first ideas with a befriended director.
But the twist you suggest is really interesting. I'll breed on it some more. Thanks again!

When a compassionate psychiatrist realizes his mentally ill wife is demon possessed, he must find a way to exorcise her before she kills him.

Then build up to the 3rd act dilemma of kill the one he loves -- or be killed.

Or pull of a 3rd Act Discovery-Reveal a la "The Sixth Sense" as Richiev suggested. Whatever. Depends on what you see as central to your theme: the delusion or the dilemma.