Killer grimaces aside, this movie seriously sucks, don't watch it. Except the part where it (sort of) disassembles everything you know about the nature of morality. You should watch that part twice.
The Secret Message
Early on, a character named Goat is bitten by a demon. He starts to turn into one, because this whole script is clearly a zombie movie that was hastily rewritten into an Aliens knockoff. When he realizes that he's DOOMed, he decides to save his friends by slamming his face into a wall until he dies.
At least I think that's what's happening. They kinda forgot to light this scene.
Later we learn that when a demon "infects" you, it can go one of two ways: If you're an inherently bad person, it turns you into a monster, but if you're an inherently good person, it gives you superhuman reflexes and a healing factor. According to the movie, this is because it accesses the genetic code of your soul.
This is where s**t gets real: Goat was genetically predisposed toward being a bad person, but he was willing to sacrifice himself for the greater good. Karl Urban's John Reaper, on the other hand, says he's "done terrible things," but when he gets infected by the magic-demon serum, he gets really good at playing first-person shooters and saves the day. That means that Reaper is a good person on the inside (promise!); he just doesn't do anything good. Goat (some side character) was born s****y, but overcame that shittiness to become selfless enough to make the ultimate sacrifice by head-banging himself to death.