One of the songs, "Maxwell's Silver Hammer", involved a student of medicine named Maxwell Edison murdering everyone with a hammer. The song is performed by Dr. Maxwell himself (Steve Martin). While we are shown no footage of a younger Maxwell killing people, it is confirmed in the comic adaptation that he was, in fact, the same guy. Now he uses his silver hammer to turn old people into younger slaves of Future Villain Band. Make of that what you will.
Slapstick violence.
A brief fistfight, some struggles, and two instances of the main character being knocked unconscious.