The literal interpretation and alternative ones
Alan “Mollymauk” played by Roddy McDowall is an intriguing character, which has spurred various interpretations of the movie. There’s the literal position wherein Alan is simply an extraordinary student prodigy. This is the view I favor based on evidence in the movie, which I share below. A couple of other theories are:
The “It was all a dream” interpretation, which postulates that Alan was just an imaginative geek who fantasized the events of the movie based on his crush for winsome Barbara Ann. As the story unfolds the fantasy gets increasingly absurd and overblown. His imaginary wanderings end with him wallowing in a jail cell as a persecuted misunderstood genius. In this view Alan becomes a pathetic martyr who’s such a nerdy loser he can’t even obtain the coveted babe even when he has the power to grant all her wishes.
The problem with this theory is that there’s no indication in the film that Alan was imagining (or dreaming) the whole thing. And, secondly, the purpose of fantasizing is to BE something/DO something/OBTAIN something you can’t in reality. So, if the story was really Alan’s fantasy, Barbara Ann would love him with undying adoration. Moreover, he wouldn’t have become a hysterical mass-murderer and he wouldn’t have ended up in prison.
Another position is that Alan wasn’t human at all, but rather was some kind of spirit, like a jinn or a fairy. Roddy McDowall (Alan) was 36 years old during filming playing a senior in high school and this supposedly goes along with this interpretation since Mollymauk is intended to look old because he’s an antediluvian spirit who possesses magical abilities, like opening doors, hypnotizing people and forcing them to do his will. Alan’s relationship with the principal is a good example: the principal is essentially Mollymauk’s puppet. Speaking of that nickname, no normal high school dude would willingly take such a silly moniker. This ties-in with Alan being an ancient creature from another time, like that extinct bird. He gives Barbara Ann everything she wants as if he’s playing with a pet, providing a treat.
This view claims further support from the film, like Alan’s “magically” appearing at Bob’s house, suddenly being tight with his mother and surprisingly living there. Wild assumptions are claimed, like Mollymauk having “complete control” over people and manipulating Barbara Ann’s mother to commit suicide. Furthermore, Alan’s parents & home life are never shown and he mysteriously knows Barbara Ann’s name and abode before meeting her face-to-face.
Here’s the literal interpretation with evidence repudiating the notion that Mollymauk is a magical spirit:
While McDowall was 36 during filming, he pulls off the role of a high school senior because of his youthful looks and the B&W photography tended to hide his age. His character, Alan "Mollymauk", is intriguing and comes across as a mixture of Svengali, Professor Higgins and Faust after his bargain with Mephistopheles. Some have described Mollymauk as a nerd genius. While he's obviously a brain, he's not a nerd because he's too cool, confident and aloof, almost condescending to those he considers lesser, which just happens to be everyone, teen or adult.
The tagline for the film is: "It's about a man living in our insane world who suddenly goes stark raving sane and commits mass murder." What brought about the total DOWNFALL of this extraordinary individual? His obsession with the beautiful-but-shallow Barbara Ann, whom he had the power to grant every whim, but couldn't make her love him. Being a virtuoso Brainiac in high school, while a gift, is also a curse socially. Alan was helplessly attracted to Barbara Ann, but he knew she wasn't the type of girl that would go for him. Still, she wielded womanly power over him and “it was beauty that killed the beast” or, in this case, the amazing boy wonder.
As valedictorian of the class, Alan was honored by the faculty and entrusted with the keys of the school and there's nothing mystical about using keys to unlock doors. He may have used his expertise with hypnotizing to gain this trust, but this 'power' didn't seem to be anything supernatural as witnessed with his attempt to hypnotize Barbara Ann, which came across as joking around anyway.
That fact that he resorts to enraged emotion at the end and mows down Bob AND several others at the graduation works against the theory that Alan was a spirit being, not to mention landing in prison.