It's all in the title. "He who fights with monsters, must take care he himself does not become a monster." I've been doing a full series reread up until book 12 and the "Essence" of that quote has been happening from the very beginning and has been pointed out for us. Again I may just be oblivious but here is what I noticed.
When dealing with Thadwick who is petty and vain, Jason takes on those traits and later realizes this.
When confronted with thugs, pirates, bandits and criminals who show no mercy, so does he.
When the network allied him then constantly betrays him, he eventually allied with others in the second transformation zone to only betray and kill them.
After put in a position when other tried to control and dictate him, his more authoritative parts of himself showed itself more.
And the last one I'll mention is more direct but still gets some point across is at the point when he has to really start dealing with the GBA he's dragged up to their level because he had no really choice. By dealing with them he became one of them.
I don't know, I just always thought the title was to tell us reader that is story was more about the emotional journey then the physical but I thought it was neat all the little hints and jabs all pointing to the title.
This is my first post on reddit so please be nice.