A real life horror movie played out on a Virginia family's doorstep ... captured in harrowing doorbell footage showing three people in creepy costumes threatening to kill them -- and even allegedly trying to break into their home.

The chilling video -- taken Tuesday night in Alexandria, Virginia -- shows the trio taunting the homeowners to open up and pounding on the door. Shayla Whiteside via Storyful

Police say the masked suspects threatened bodily harm to the family ... saying they'd kill them -- and demanding to come out or they would come in.

The victim -- a woman named Shayla -- was home with her mother, younger brother, and their family dog when it all went down. Shayla Whiteside via Storyful

Shayla told WUSA she thought it was initially a Halloween prank, even respondin

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