User blog comment:Mrlmm0605/Let's Talk Bleach! - Part 2: Hollows and Arrancar/@comment-26091912-20150607095302

Hollows are collectively evil, not because they follow their instincts, but because they devour human souls and (the more depraved ones) each other just "for the fun of it". Some people become hollows after death, though this begs the question of whether those who became hollows did good during their lives as humans, or at least whether they were selfish. Regardless, devouring souls does not seem to be necessary for their survival; in the case of greater hollows, they will regress into lesser forms if they don't continue to devour other hollows. This "fear of regression" is what impels hollows to devour souls.

That is not say all hollows are inherently evil, just that they remain evil so long as they continue to pursue their carnal lusts without remorse. This is especially true of arrancar, as they are the most sentient. If they continue to senselessly kill other hollows/arrancar, then they cannot be said to be good. Senseless killing goes against the conscience of all beings with higher reasoning.

You could argue that Aizen created and manipulated the arrancar for his evil purposes, but it's not like he threatened them upon pain of death. They followed Aizen of their own volition, and if their will was so aligned with his, then they were no less culpable of murder than he was.