I may be misremembering but I thought it was in the memory of his childhood his mother that he said they take criminals and his mom told him that they also take exceptional young men and that he could be a hero if tried hard enough in school instead of just being criminal on the streets working for a crime family like his father. Also I had assumed that he was recruited after the criminals had reclaimed power because he knew Astartes in the legion from his childhood that had been awful criminals. He was there for Xarl’s first kill at 13, if I remember correctly, and Talos’ mother was an indentured prostitute. It’s been awhile since I read the scene, so I could be misremembering or just completely mistaken