A recently released psychiatric patient named Tom Sykes creates a home for himself in the attic of the Dreyer family's newly built house. He uses electronic devices to spy on them. The Dreyer's beloved dog Rudolph is murdered by Tom in an attempt to defend his family from the titular hider. After that, Tom really begins to get down to the business. He starts to focus his attention on the mom, Julie, going so far as to watch her skinny dip in the pool. He interferes secretly in the relationship between Julie and her husband, Phil, planting bogus evidence of secret love affairs. He befriends the Sykes' son Neal and teaches him fighting techniques. After two explosive arguments, Phil leaves the house and moves into a hotel. Seeing this as an opportunity, Sykes pretends to be a visitor who lives on Willmington St. a couple of blocks over. His attempt to insinuate himself into their lives works at first, although the creepy neighbor (and huge Philly fan) Gene is the only one who distrusts him. Sykes murders two people who had accidentally discovered his bizarre goings-on and finally Julie becomes sufficiently suspicious to reject his advances. Tom loses it, and he tries to kill Julie. Phil shows up and is severely injured defending her while Julie gets a gun and shoots Tom in the chest. This does not kill him, and Tom gets up and tries to kill both Julie and himself, but the police (summoned by Gene) shoot Tom dead from the doorway.

Question: Why did Tom cause conflict between Julie and Phil?
Answer: To attempt to break them up so he could integrate himself into Julies life.
Alternative answer: Because he wants Julie for himself.