Destroyed four lives?
At the films climax, just after Amanda has shot Lynn, John sais "You've just destroyed four lives..."
Who were the four?
I only count three.
One is Lynn, obviously because she's about to die.
Two is Jeff, because his wife will die and he will be a widow.
Three is the daughter, who even if she did survive the room (which she did) her mother would be dead.
Those are the three lives that have been destroyed.
He couldn't have been referring to Amanda, because he didn't know Jeff would kill her - the whole purpose of his game was not to act in revenge.