[kayote]

i don't know if this will help with the Arrencar being good and some being bad

As the hollows go from Menos whom are just foot solders to some of them having some sort of instinct to become stronger they feed and than if they evolve into adjucas(can't Spell) they gain more traits like sort of emotions that manifest as they get stronger and they have a moral sense from that (i'm just gessing hear from the past things that i have read and adding my own)

like when Grim was with his gang before he became an arrencar Showlong was saying how he should bite them to evolve because he knew that this is the strongest they were going to get, whereas Grim still had the hunger to become more than he was. that is all human emotion, it may be that it is to do evil things but, he still wanted to become more than he was.

then if they become Arrancar they have a Shinigami side and that gives them more charcater traits and sort of a moral code to each of them. as they are all uniqe in there own way. that could be a reason to why thay are all different and the way they do.

also as they experience new things like fighting different people who have different openions of the world they could be learning and become influenced by other and there way of life. as they have already evolved so much and they have developed all of these emotions and feelings about different things that could mean that they are sort of adopting different values that each of them see as right and wrong.