As I have been writing my script, I've noticed that the real "evil" in my story, isn't the killer. But his weapon. Anyone who come in contact with it becomes a little like the killer.
I am thinking of making this stronger in my second draft. Any ideas on how to make the connection to the audience without a character explaining it. I would like a smart viewer to figure it out before the heroes do.
For reference: The item in question is a sword.

http://therathskellar.blogspot.com

Maybe you could show the effect the sword has on it's wielder by being clear in how the killer is when he/she has the sword and when they don't. Maybe there is a major visual change - darker eyes, a toner/leaner build, or a terrible switch in their personality - a la Jekyll & Hide , etc... I guess it depends a lot too on the setting and how much you can get away with.

This recalls Elric of Melnibone when I think of this. He was a weak, frail character without his sword, Stormbringer, but when he had it in hand, he was stronger, virile, healthier, and unstoppable. If I remember right, the weapon actually sustained his life - like life support. Might be good research to see how Elric was handled.

Hope this helps.

I would go with cylothrop's ideas, but toned down a bit. Instead of a major visual change, just change their posture a bit. Instead of a terrible switch, make the characters become slightly more irritable or sadistic. If you want the smart people to figure it out before the characters, and you don't want the smart people yelling angrily at your characters until they figure it out, you have to make it subtle.

I think the best thing to recall would be The One Ring. Ensnaring, sneaky, devious and subtle. It eats away more and more at your psyche the longer you are in possession of it. Think of the killer as Gollum and the other characters as Bilbo or Frodo.

Have a change in personality. Have the character yelling more often, or have them just be mean to their friends for no reason. Change the character completely.