One thing that doesn't have sense
I like this movie very much, but there is one illogical thing: as Crispian says at the end, it was so important for their plan they keep Erin alive, to testify in his favour, so why all the killers try to kill her? They should've left when all the other family members were dead, but they stay in the house trying to catch Erin, which doesn't have any sense, and goes against his plan.
The movie works anyway, but thinking a bit about it you see it doesn't make sense.