Demon and Horror, what is the name of this movie

The beginning of this movie starts out with a sex scene with a man and a woman, the man kills the woman, the police pick him up and put him in prison, he uses his teeth to rip a bunch of skin out of his arm to get to a medallion that turns him into a monster or demon and then he goes through the prison killing the other prisoners. Other things I remember about the movie: The woman is basically wearing a school girl outfit, the symbol on the medallion is some kind of star, I think the man kills the woman by stabbing her with a knife (or transform and kills her), when the police get there the star is drawn in blood on the wall inside a circle.