The Tree of Parrots - jegulus
4 parts Ongoing MatureRegulus black getting in vicious cycles while grieving his brother, and being constantly attacked by a classmate.
James potter helping him as the same man is pushed off a cliff, and the situation quickly degenerates into insanity.
From 3rd chapter:
"Regulus couldn't help but attack others, the more he tried, the more he found himself distancing from everything and everyone. Maybe he wasn't meant to connect with others, maybe this wasn't for him. Maybe the distance from people wasn't a choice, but a natural condition."