The 'real killer' may not be the one? (Spoilers)
I didn't mention Mrs. Spool in the title to avoid giving away spoilers to those who haven't seen this. Could there be a chance that Norman is actually the real killer rather than Mrs. Spools? Remember in Psycho III Statler the diner manager says Spool never missed a day of work and 2 of the killings happened during daytime(the boy and Lila Loomis) not to mention the phone calls. None of these could have happened if Spool was at work. Besides, why not Norman? He was doing all the killings in part 3 and at the beginning when he first got back home he already had a hallucination of his mother talking to him. Who's to say that Norman couldn't have killed anyone before Ms. Spool came to the house at the end? He was crazy enough then to kill her and talk in her voice the same day Lila was killed.
Some interesting thoughts: assume that Norman killed no one until the end, would the killings in Psycho 3 have happened if Mrs. Spool never went to Norman's house at the end? Then there would be no "Mother" sitting at the house in part 3. Would the people in part 3's timeline have been safe if Spool, Mary, and Lila never bothered Norman in this film?