In general I'd say that pulling a gun on somebody for wearing a "serial killer" Halloween costume is a bad move. But for Sydney specifically...it's not just "some serial killer", it's "a series of serial killers trying to kill her specifically". If she's home alone and someone bursts out of the closet wearing this costume and wielding a knife, just statistically speaking she's better off shooting first. It's theoretically possible this is some rando pulling the dumbest prank imaginable, but the odds are pretty good it's kill or be killed.
EDIT: And if it is a prank, I doubt she'd be convicted. It's like charging a cop while waving around a real-looking toy gun.