"So go to the scientists, fool! We have never seen an abomina- half wingly before, so how could we possibly know what to do? Oh, and if you want to die, you can sign up for it on the piece of parchment to your left."
Thanh discovered that you don't choose the spirit, it chooses you. But how could it know who to choose? Thanh figured that maybe it has to do with the personality of the person. That man had a personality very describable by darkness. Thanh thought that he needed to get more.