with the delivery guy toward the end. He walks up to the door, she's screaming for help, and then all you see is his arms fly into the air, she screams and all of a sudden there's blood on the door? WTH? Did he die? and if so, how? The killer was inside.
Bill knows that store like the back of his hand. He must've heard the bread delivery guy pull up because he disappears from behind Jennifer. Then, the guy's at the door and splash, dead delivery guy. Yeah, Bill got him. But the version I saw used jump-cuts or something to hide some of the gore. Like you said, his arms flailed, then there was a huge blood spatter on the doors. Haven't seen the Unrated version yet. Maybe that scene is extended a bit.
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If something's too good to be true, it's best to shoot it just in case.