IMO:
The Killer is a societal outcast. He doesn’t date and doesn’t get invited to parties. The “Party House” has been a source of aggregation and rage for a long time. It is less than 150’ feet away. He would move … but for his one year lease.
The loud party laughter goes into the wee hours and cuts him three ways.
1.) It deprives him of sleep. (2.). It is a constant reminder that he is an outcast… no friends … no fun. (3.) Occasionally he catches a glimpse of the girls. He fantasizes about them. It is a love/hate thing.
The killer has a variety of knives including two Rambo knives, a bow with arrows and several guns. In his mind, he has killed the girls many times. His killing fantasy involves a knife because it is PERSONAL!
That night is louder than most. Finally, he falls asleep. Then, at 1:45am they come home and he is again wide awake. That’s it! He waits for their lights to go out, puts on his black ninja outfit, rubber gloves and slips outside. He is going to make them quiet … once and for all.