Maybe he would be nicer, I really got image of him as a good kid in beyond when he and gran sit around campfire. He was cute and he had dreams and hope.
I think 60% of his current character was because of Gran teaching, this old woman was so devastated after lost of her homeland and people and she pass that everything to Nezumi (not like you cam blame her for feeling this way). Her hate and way of thinking influenced Nezumi a lot.
Second thing that changed Nezumi really deep must have been hunt and Correctional Facility. He was like 10? when he survived that hell.
Last thing, living in the West Block, alone, with no help, no one close for 12 must have been hard. How he managed to get food? He wasn't actor at beginning for sure. He was hurt by people many times probably until he find his way to survive in such environment.
Would Shion be able to get back Nezumi as like when he was kid? It's hard question, some scars are impossible to heal, mental too. Nezumi gone away in most important for that moment I think. Nezumi again has to cope with a lot of things, second hell and he chose to do it alone.
Maybe Shion's love wasn't enough to fix Nezumi's hurt?
Damn, I should go to sleep because I again starting to analyse them too much and it turn into dark tones... XD