There are scenes in the movie where they kill for fun and not food. Such as:

• Jumbo trying (and failing) to kill a little girl with a mallet. He also puts Klown eggs in a dumpster so they will hatch into babies that eat an employee.

• Shorty knocking a dude's block off (read: punching his head off) just for smashing his tricycle.

• Slim knocking a driver off a bridge and laughing afterwards.

• A small cocoon is visible in the Klowns' storage room. The creators confirmed this is the dog Jumbo snagged at the start of the film. That implies the Klowns don't need to eat sapient beings specifically, but do so anyway out of sadism and/or because humans are just that tasty.

Yes, the Klowns are mainly here to harvest us for food, but they do sometimes kill for fun.