Well, he's not a mad man. He's Joerg Sprave, a very nice guy who built the slingshot of mass destruction among many other deadly slingshots. In his own words:

Circular sawblades are about as dangerous to be shot from a slingshot as it gets. Heavy. Sharp. Unpredictable in flight.

It turned out quickly that rotational speed is needed in order to stabilize the flight of the blade. Otherwise it will tumble and loose speed quickly, as shown in two slow motion scenes. So a special weapon was devised that adds this rotation to the saw blade, and the effect is quite impressive.

A shot of the blade into a watermelon demonstrates the destructional potential of this slingshot invention

OK, he's a mad man. But we love him all the same.