(Spoilers) The answer to the big question is so obvious
So people (including the friends with which I saw this movie) want to debate whether he actually killed his other victims.
I never understood this debate.
Of course he did.
If he let all the other people he tortured live SURELY by now he would have been discovered. Surely by now SOMEONE would have told the police. I mean if you go through something like that and wake up on the street, wouldn't the natural impulse be to run to the nearest police station, covered with fingerprints, hair, DNA, etc.?
So I just don't get this debate. He let John live to get the money because he knew John was implicated in a crime and would therefore be wary about showing up in a police department to tattle about what happened in the house (of course, that didn't fully pan out, but the reasoning is very clear).