Without actually telling what you are spoiling.

No, John. You are the demons.

It was a dream all along.

"No... I am you father."

Toy Story 2?

I'm the bad guy but I can't remember?

@JediPrincess
Sounds like Total Recall to me.

@Thorvald
Oh, I know! It's from some obscure sci-fi movie from the seventies.

@chaseawaythedark
Let's say, Super Mario Bros. 2

I see you are a man of culture as well..!

I was gonna say KOTOR.

Marketed itself as a fairly accurate adaption to a comic, only to make the original protagonist disappear at the end of the first episode then have the story play out like a "what if" from there.

The only winning move is not to play.

@Thorvald
Hm, or it could be this one Edit: ah yes, forgot that there is a decaptiation in the scene aswell, so gore warning!

@Cynicallia As said by HAL 9000, when it got owned by Dave in chess (deleted scene)

@Rinnarrae (Not sure what you are exactly referring to, but) the first Mission Impossible movie from 1996 did a similar dirty trick with the main protagonist from the old TV series.

Either you die a Hero or Live Long enough to see yourself become a villain

@dustsucker2099 I'll give a hint that it's something that came out recently, and I almost don't want to say what it is as it's something that's best watched blind with the exception of the source material.