Not a spoiler, just my theory

So, my theory is that Little A killed the chicken from his memories and that he did, indeed, have antisocial personality disorder. People with this disorder show signs early in life, such as lack of empathy, harming animals, etc.
Probably his parents took him to the farm thinking that a change of scenery would calm him down, but it had the opposite effect. So they took him back and signed him up for this program, so he could live like a normal person.
However, the program went wrong and turned all test subjects into extremely loving people who were overly willing to help others, which caused confusion in the subjects’ minds because it was the opposite of everything their instincts told them to be (also, as a fact, antisocial personality disorder has a neurocognitive component, as if the brain had been programmed that way, so to change it completely you'd have to change the brain's structure).
That's why Little A suffered as a child in the city, and why all the other test subjects unalived themselves.
I also think that's why even though Little A is so eager to help, he also has moments where his "helping" goes overboard and lacks of empathy (like wanting to help Kendrick to lay eggs, but also forcing him to do it trough sex even though K's clearly uncomfortable).
And so, the reason why his uncle is in such a rush to come back is not because Little A's in danger (as I thought in the beginning), but because he could hurt the people living in the farm.

:)