The reason the killer wears a mask, is because the he plans to frame another person (Bobby Slade) for the murders. He did not kill his first victim - Julie Jensen - whom he attacked in the same ritualistic manner as the murdered Mrs. Forrester. Julie Jensen would be a witness and implicate Bobby Slade in court. Bobby had slept with both women. Also, the killer - Mr. Forrester - wears a mask to kill his wife and her maid because, by remaining anonymous, it's easier to kill them - easier on his conscience - especially if he needs to lead a convincing double life for his attorney. Wouldn't it be easier to kill people you know, and live with it, if they don't know it's you?