It's not hard to understand.

The killer with the chrome skull was a serial killer that murdered his victims (most likely prostitutes) and sent the videos to the police. This is explained when that guy finds a file on him on the FBI website. The killer, for whatever reason, takes his victims to the morgue and puts them in coffins. He has some sort of deal worked out with the mortician/caretaker where he pays him to let him use the place and to keep his mouth shut. Somewhere along the line, their relationship crumbles and the caretaker wants more money or wants out of the arrangement. This happens on the night he targets The Girl, a prostitute, by taking her to a motel room and knocking her unconscious (probably his M.O.) and putting her in a coffin. The caretaker also doesn't help her when she's screaming for help in the coffin, suggesting he's willing to look the other way. But after his fight with the killer and a demand for more money, chrome skull dispatches him. Everyone else killed in the movie is just out of pure circumstance, as he's just trying to get to The Girl. He even tells her "Many people are dead tonight because of you." He kills everyone in his way to get to her. He's not supernatural or superhuman in any way...the bullet he takes does hurt him, but he performs surgery and sews it up himself. The meaning of the mask he chooses is irrelevant and unnecessary. It has no importance to the plot--the mask could have been anything. And why he's doing what he does...well, he's a serial killer. No need for further explanation.

I don't see how it was that hard to understand, LOL.