Responses
I feel like you have to reread the 1st story. He wasn't hunting down witches, he only needed one witch to form a contract with his father that was dying of cancer. The uncle (who shot Keigo's father) from their organization was the one hunting down witches for money.
Keigo and Natsume are both interesting to read about but it still does not make sense to put other people through the same things that you went through yourself, but I guess you know what they say, hurt people hurt people. Natsume hunting down other witches when people were targeting him for being a witch is a big hypocrisy on his part.