It’s about Order vs Chaos.
Some Diablo 3 spoilers.
Tyrael makes it clear in Diablo 3 that Angels cannot choose to be anything but good.
Ok, explain Izual? Explain Imperious?
No, the Diablo series is about the extremes of Order vs the extremes of Chaos. The Order side is the Angels. Izual presumably saw a path to order through working with Hell. Imperious is imposing order at any cost.
This doesn’t necesarily mean that the Angels and Demons actually understand this. They are doing their thing, making value judgements on it. It’s the humans, the ones who are capable of choosing which path, Order or Chaos, they walk, who can truly see what is good and what is evil in these two paths.
This is where Tyrael as Angel of Wisdom, as a mortal human, comes in. He alone among the Angiris Council can understand that Order and Chaos should not, indeed cannot, be at war if anything good is to come of it. They must be in balance.
Perhaps Leah, if anything remains of her soul, will take up a similar role in Hell?