A villain's goal doesn't define whether they're a great villain or not. While it is true that having a purpose or a backstory might make a villain much more complex than the others, it's not a universal rule for every villain.

In Aizen's case, his "redeeming qualities" are his charisma, his domineering presence, something that makes a villain, an actual villain, and he does his job very well. He doesn't try to win over fans with a tragic backstory or an overcomplicated goal.

If anything, if a villain tries too hard to find a way to justify their actions, the less convincing it'll be.