The movie was a grim view on humanity to begin with. But my main issue, which will be of no surprise to anyone who has seen this movie is the beyond stupid ending. Here is why.
1. No one should ever play that sick of a prank on anyone. You are making them think that someone has abducted and murdered your closest friends and girlfriend.
2. The film ACTUALLY shows these people being killed and mutilated. So how it can go from that to "prank" is unrealistic.
3. What did they expect would happen when they set up a scenario that has someone kidnap, torture and murder his friends and girlfriend and then put him alone with this same man in a room? How would any sane person react if put in that situation? Hell even if someone had only murdered my girlfriend, that would be enough to push someone over the edge and react violently towards them. No one should have been surprised at his reaction which was, at least from his perspective, to kill the man who had already killed all his friends and girlfriend and who was most likely going to kill him too so there would not be a witness to these apparent crimes. They placed a person in a situation where he would have felt like he was in a fight for his life. He did what any person would do in that situation.
4. Considering that this room he was in was filled with the still alive people who were all in on this prank they all watched him attacking and killing this man and did nothing to stop it. In the real world every single one of them is an accomplice to murder and would all be going to prison, even just for being part of the prank as they orchestrated the events that lead to the killing.
This is a movie made by some very sick minded people. If they were attemtping to create a shock ending then i guess they succeeded, but not in any good way.