A Jerk, An Idiot, and a Psycho
To me, this is a movie about a jerk, an idiot, and a psycho. Simon is the Jerk, the wife is the idiot, and Gordo obviously is the psycho. I couldn't bring myself to "root" for any of these three, though I don't think that we are meant to.
Simon is a jerk and a bit of a bully, but not the demon he is made out to be on these message boards. This is the USA, home of Football, boxing, UFC, wall street, & big business/capitalism. Until just very recently, being a bully was practically universally praised in this country. (I'm not saying this is right or my personal beliefs, just showing that people like Simon tend to be a bit 'normal' nowadays)
Robyn (I believe that's the wife's name) is an idiot. Any woman who lets a unknown man, or a man you barely you know, into your home when their husband (or at least a friend) is not home is a fool. That should be basic "Women survival 101" stuff. Also, while their marriage was clearly far from perfect, the fact that she repeatedly sides with this bizarre stranger over her own husband was shameful.
Gordo is an absolute psychopath. The fact that so many people on here are sympathizing with him and equating him with Simon is truly frightening. He is a stalker, broke into their home, broke into at least one other home (that we saw), drugged his wife, likely kidnapped the dog, and possibly raped his wife.
To me, the take away from this movie is that being a jerk is bad, but being an idiot or a psycho is worse. Just my thoughts.