Here is my take.
Mephisto-Lawful Evil
Baal-Neutral Evil
Diablo-Chaotic Evil
Andariel-Neutral Evil
Duriel-Chaotic Evil
Belial-Lawful Evil
Azmodan-Neutral Evil
Thoughts?
They are all chaotic evil.
Every single one. Everything they do is based purely on their own desires. Choosing to work with another demon to further their self interest is still chaotic evil.
It is only the angels who have alignment diversity. For example Imperius is Lawful evil while Tyrael is Lawful good.