Leave a lot of things about them up to the imagination. It's very difficult to make someone you know in a story scary. They have to be a stranger, someone we know isn't going to be scary unless we really know their personality and then they change drastically and become a stranger again.
Subtly emphasize the notion that their politeness/friendliness isn't real. Make sure the reader knows that they can be dangerous, but leave the extent of that danger up to the imagination. If he's a slasher, then imply that he's a slasher, don't leave it out in the open.
A scary character doesn't run up and stick a knife in someone, because that's not scary, at least not early on when you're introducing the character. You have to let the crazy burn slowly. As opposed to stabbing, scary character wraps his arm around somebody's shoulder and says, "Like this knife? I found it a long time ago in Vietnam. Beautiful piece of steel, don't you think? Y'know, why don't you keep it? No, really, I want you to have it. I don't get a good use out of it anymore."