A good start is obviously with a Santa suit. If you own the suit and are not renting it I'd suggest dirtying it up a bit. I also always thought that when the beard was pulled down under the chin it made the Santa look a lot less like Santa and more like a psychopath.
Add some dark makeup under and around the eyes, add some scars and some blood and you're good to go. If you have the means gets some red or yellow contact lenses as well. Don't forget to top it all off with a blood smeared fake ax and maybe a present bag with a fake arm sticking out of it.