Allow me to explain the ending to you guys who are confused..
Been reading the posts on here asking all kinds of questions. Like did Warwick actually kill his other guests or why did he let John go. Was it the chess game he won? Etc.
Look I think the answer is starring us all in the face but the movie doesn't really convey it. Warwck is a cop right? He knows John is connected to a bank robbery where he just scored 300K.
So......what is there to gain from killing him? He would never see the money. Even if he played a part in catching the girlfriend there is no way he could just take the cash while the other officers were there assisting with the arrest.
He pretty much let John go with the hope and somewhat risky plan of getting to that money. He only took 1 pill in the morning because he needed to be "focused" remember? Was it calculated or could Warwick have possibly known he would have been put in the right place at the right time to get the money? No not really, but not letting John go meant 100% no chance of seeing the money.
The chess game does come in play because it revealed John's intelligence and ability to calculate your opponents move. When the officer informed Warwick that John's GF had double crossed him Warwick made a judgement call. He knew John would somehow find a way to get the upper hand on the GF and when that took place Warwick knew he needed to be there.
Yes he killed the previous guest and most certainly he will kill the detective when he comes to dinner that evening. They have nothing to offer. Now the film isn't perfect. Why did he dump John's body in plain sight or why did he have the makeup kit for fake gore?
Well the dumping of the body thing I cannot logicly explain.
As far as the makeup goes it's my theory that Warwick never actually cut any of his victims. Come on, this guy couldn't stand the sight of blood or any kind of mess. I think he applies the makeup to his victims to scare the hell out of them and screw them up mentally. He then takes pictures to scare his next victim. He strikes me as the type that would just kill them via drugs rather than make a mess and get blood everywhere.
The film didn't really do a very good job of fleshing out my above theory, but I think it's the answer to the main question about the film and to me it seems clear a day why he let John live. If he killed him he would never see that money.