Why wasnt Rawlins arrested?

For starters all you see of him and the end of the movie is him standing over Sadie's body, so who really knows.

Second, the other detective is well aware of him being close on the trail of this mad cult of psychos or whatever, and could end up vouching for him in the end. Remember as movies have it, once a reputable cop speaks up for another cop, however far removed he may be from the department, the second cop normally has a pretty decent fighting chance.

Third, as the only survivor of the whole thing, who's to say he can't fabricate some story about what really happened. You have a a couple of dead bodies, on top of the bodies of two women ALREADY known to be dead. Easy sell in my book. Crazy guy Bishop thought the dead girls were vampires, stole the bodies, did them in with silver steaks. Rawlins removes the stake from Sadie in order to fight Bishop off. Case closed.

Also remember - he called for assistance. When they get there they find he has offed two dangerous members of a cult - a cult that has lots of bodies hanging upside down in the next room with their throats cut. He has obviously been attacked, his kevlar vest shows he was shot. It would not be difficult to explain how he fought and managed to kill the two cult killers with the silver bolt. He's probably going to get a medal rather than arrested. How he might explain how his daughter and Sady who are already legally dead came to be running around and being killed again is another matter.