Nuh-uh, they become a Witch once they fall into despair and get depressed. And people fall into despair because of being sad. That has nothing to do with it.
The Grief Seeds are supposed to make them power-up and become stronger, sort of like with the Power Cards in Shinzo, or the Dragon Soul Energy you gain from defeating a Dragon in Skyrim. You know, like a power-up you gain from beating a boss and enhance yourself with it. Like with each Witch they slay, the stronger they become!
You have it wrong there.
Um, that's not what I heard from the series.
Also, have you ever seen "Shinzo," or played Skyrim at all?