The killers motivation (spoilers*)
The mute and his buddy wanted to surprise/scare her, then they had to hunt her down and kill her, to prevent her from talking? Ok cool fair enough, but why bash her with a hockey stick, what was their relationship with the older brother, the mutes buddy really seemed to have some hatred, was there some homosexuality going on between the two younger ones, and they were actually jealous of the older brother?
The writing needed way more development there.