Just the idea that there's a masked giant in the middle of the woods, watching you... waiting for the right opportunity to stab you with a machete or cave your head in with an ax.
Crazy Ralph's rantings about a "death curse" may be — well, crazy — but the franchise proves he's not wrong. If someone does anything around Crystal Lake, even if every single person with the last name of "Voorhees" in the area is confirmed dead and buried, something will go pear-shaped and a lot of people will die gruesomely.