Now that the servers aren't playing up, we can try getting back into this

What I can tell about the prisoner's song choice is that:

He hates the non-human DNA inside of him because of the prejudice it could bring, demonising it as monstrous and desperate to be accepted by the ones that would discriminate him or view him as 'unworthy' of their version of salvation or paradise.

He is very determined to go against 'the world' (AKA the world that accepts and respects non-humans), despite feeling powerless in the face of the opposition. This likely sparked a need to gain power by joining with those of similar mindsets, like the City of Claus.

My character, given his experiences, would likely know/ be able to infer this much but, given that he's not a trained psychologist, anything more detailed would probably be out of reach for him.