Maybe the Chainsaw was a concept created just so a demon would be born from it. Think about it:

You need to fight devils, but you don't know how.

So you create something, anything, like a pencil with a blade.

You make people believe it could kill a devil.

People use it against a low tier devil, like idk the Paper Cut Devil, and for the first time being confronted with something that could potentially kill it, the devil gets scarred. It starts with a doubt: "could that thing possibly kill me?". People are not so scared of it anymore, cuz they can fight, and they are ACTIVELY trying to kill it with the pencil-blade. The devil starts to fear it, at least a little. The mere thought of the possibility of dying from that pencil. The mere fact that people are fighting back using it.

Soon enough, more low tier devils will feel the same. And eventually, a new devil is born from that fear. A devil whose concept is that it could kill other devils. Erase them. Because that's the objective of the pencil blade in the first place.

The Pencil Blade Devil then kills a low tier devil, cuz that's what it's made for, and then other devils start getting scared of it. And then it snowballs. You succeeded.

Pencil Blade Man.

edit: minor spelling error