I totally did not understand the end of The Shortcut?

So I'm talking about the 2008 like horror movie and i just didn't understand the ending. The only part I didnt get was when Derek's brother killed Lisa and Derek seemed to not care at all even though he was freaking out when the old man tried to kill her. Also I didn't understand why they killed every main charecter in the last 5 minutes of the movie. Can someone help me?