Angra Mainyu says “This is what you get for showing kindness to the devil” and what he means to say is “It is pointless to extend sympathy to evil. You will gain nothing of value from cherishing villains. Look at this worthless strip of cloth. For all your efforts, this is all you will get from me.”
Except what he ends up actually saying is “I have lost my name, my face, my everything. This is the only thing that truly belongs to me. That is the one thing I can claim as my own. And I want you to have it.” This is what you get from showing sympathy to the devil.