Continuity mistake: As Tommy reaches Jason's grave, he hears laughter and voices coming from the woods. As the voices move closer we see lights in the clearing. Tommy runs off in the bushes to hide. Moments later, two guys with flashlights and shovels suddenly rush in, from the opposite left, instead of the clearing path at the right, from where they were.
Continuity mistake: While Eddie is being killed, in the second shot, there's a blood driblet on his bottom lip. In the next shot, his bottom lip is clean without any blood residue.
Continuity mistake: When Lana is hit by the axe, it hits her horizontally, but in the shot of her corpse, it is shown vertically.
Continuity mistake: After Pam runs out of the house towards the barn, she sees Roy coming, and then she trips, as she crawls her shirt sleeves are rolled up, but they weren't before she tripped.
Continuity mistake: In the film Demon is in the outhouse and is sitting on the bowl when it begins to shake. The toilet paper changes amounts on the roll between shots. (00:53:00)
Continuity mistake: The sound that startles Lana was breaking of plates. But when she investigates, she finds the cat sniffing fallen pots. (00:34:40 - 00:35:50)
Other mistake: After noticing that Tina is dead, Eddie fearfully backs away and leans against the tree. You can see Roy put the leather up and over the tree. This is impossible. He would have to whip it around the tree, not over it.
Continuity mistake: When Demon is in the outhouse and the wall is speared the first time you can see the pike coming in above the toilet paper roll. When it cuts the pike is almost below the roll. (00:53:15)
Visible crew/equipment: As Pete walks off into the woods, the cameraman's shadow is briefly visible on his back. (00:23:50)
Continuity mistake: When Violet is dancing in her room, Jason starts to walk up to her and we see the knife has dried blood on it. Then he picks her up, stabs her takes the knife out and now the knife is clean. (01:04:55)
Revealing mistake: When Tommy is about to slice Roy's arm, you can see that he is not really holding the machete in his hand. (01:19:20)
Continuity mistake: Lana smears lipstick on her upper right lip, but when she turns at the cat's noise, it is pressed and perfect-looking. (00:34:40)
Continuity mistake: When Jake gets a cleaver smacked in his face, you see him get slashed across his face. When Robin goes to bed and she sees his dead body, you can see the slash on his face is in the opposite direction. (01:01:15 - 01:03:30)
Revealing mistake: When Pam is out looking for everybody, when they show her inside the truck, her hands don't appear to be on the wheel.
Answer: He simply just snapped. He definitely looked angry before Joey even started talking to him, and said how he hated being there...and he already had an axe in his hands...he just lost it, and took out his anger and frustration on Joey.