I don't think Bruno has any mental problems pathologically, just that there are parts of his intellect that never matured, because they were not stimulated. I think he was never taught the full range of normal social behavior, the significance of relationships, or any social bonds, including familial ones. To him, his baby is just another creature that can be used for survival. "We'll have another one" he tells his girlfriend, like little Jimmy was a pet. He has no resentment for the baby, he's just not attcahed to him, so he does the only thing he knows to do to survive: sell him. Bruno acts like someone who probably never experienced parental love or any form of bonding with his parents or anyone else. Do not be fooled by the fact that he can read, drive a car and has a cell phone. Bruno is the typical street child: raised in a rather institutional environment, without any type of affection, until the age of 18, then left to fend for himself. He might have been given food, clothing, shelter and basic schooling as a child, but not taught many social skills or how to develop a normal emotional self. He survived by trading everything he could, thinking people are just means to an end. Even his love for Sonia, although strong, is primitive and childish - it's based on instincts, not intellectual communication, and it's expressed rather rudimantarily through puppy-like play and sexual intercourse. If you noticed, he has no family and no friends, just Sonia - his mate, but she means the world to him. In spite of his actions, it's clear Bruno doesn't have a mean bone in his body. He's always gentle with Sonia, even when she refuses to open the door, leaving him hungry and with no place to go (there are enough "civilized" guys who would have broken in that door instead of seeking shelter in a box under a bridge). When his kid partner gets caught, Bruno returns the money and admits responsibilty for both of them.
Many of us have seen people like Bruno in the large cities of the world. They grow up like weeds, never nurtured by anyone, never taught how to love, how to understand themselves, develop social relationships, see life as more than just mindless survival. They're resented by "civilized" people for their "refusal to take responsibility" or "being a burden on society", etc. Like blaming a blind for stepping on one's toe...
"We are all in the gutter, but some of us are looking at the stars."