Basically Heinrich Kramer and his book. That was one dedicated guy. Some figures: about three million withcraft lawsuits in German Roman Empire, 1.5% condemned to death i.e. around 40000 people, but inquisitors actually played a role in about 1000 death sentences. Kramer himself had 200 of those. So one guy basically killed 20% of the people inquisitors killed in Germany for witchcraft. That is some "dedication".

He was quite controversial in the church. Georg Golser, the bishop of Brixen considered him a madman and tried to hinder his work (German language wiki )

It was basically the unfortunate case when a very... fanatical man got support from the Vatican. (The Vatican's attitude about withcraft ebbed and flowed.)

And then his book lived on and had a lot of influence.

Sources:

Michael Hesemann: The Black Legends / Die Dunkelmänner

Marian Horvat: The Holy Inquisition, Myth or Reality