Continuity mistake: When John Strode goes downstairs, his hair goes from dry, to moist/wet looking and hanging down.
Plot hole: Jamie takes off running with her baby. 33 seconds later, Michael kills the nurse. 30 seconds have passed since killing the nurse and Michael is right behind Jamie and somehow manages to find a knife along the way. All in a one minute 5 second time frame. (00:04:50 - 00:05:55)
Factual error: When Tommy is in the phone booth, there's fresh bright red blood on the floor. Since it's from Jamie it would be dark brown by now since it's been there all night.
Continuity mistake: As the film progresses towards the third and final act, Michael's body goes from large and muscular to lean and thinner. No attempt is made to hide this. The reason is because the director decided to change the ending but didn't bring in the original actor, instead choosing a second one whose body doesn't match.
Answer: The other answer is based on a novel and not on the film itself. According to this film (and the alternative Producer's Cut), the curse of Thorn is an ancient druid curse that must be fulfilled - a chosen one is picked from every "tribe", and made into an almost superhuman figure. This chosen one will then be forever compelled to sacrifice and kill-off his or her own family on the night of Samhain (aka, Halloween night) every year until the curse is either stopped or passed onto another.
TedStixon