Of all the numbers available in the universe, 13 seems an odd choice for the "scary number." It's just an integer - kids generally master counting by the time they're six, so they've completely conquered thirteen before they're comfortable going to a public rest room alone. Then later in life, we tell them "Oh, by the way, one of the numbers you've been working with, it's evil - avoid it at all costs. Adults do."
If you're looking for a scary number, why not something more mysterious like the cosine of e or simply pi? Pi already sounds evil because it's the only number named after a delicious snack - and that's just the sort of dirty trick the devil is always trying to pull. It's got that weird Greek symbol which looks like something you burn into a vampires forehead to make him obedient. Those two vertical bars with the squiggly line - c'mon, it just looks unlucky, like an N crashed into an F and got horribly mangled in the accident. Thirteen's just a one and a three. How low is your threshold for being scared if all it takes is a one and a three sitting together to make you nervous? And if things get scary when a one and a three get together, why isn't four an unlucky number? The movie Pi was pretty scary, but the movie 13 is about sexy jailbait - and we all know there's nothing evil or scary about that.
And why are we scared of a number, but not a color or a letter or a sound? You never hear someone say "Look Out! It's a chartreuse L and it's making that Bzzzt sound! Run for your lives!"