The diablo franchise as a whole revolved around the idea of manipulation, the prime evils were never seemed like they were after power, but rather just doing evil things in order to spread hell through out sanctuary, which is why they were always so hard to find, an actually perfect example of this was Belial of all demons, he hid in the shadows and corrupted from within turning the citizens on each other, he used his position to completely control the country while looking innocent and helpful.
It is what Diablo did with King Leoric and what Mephesto did with the Zakarum priests.
Ba'al manipulated Darius into believing what Tyrael did was wrong and to free him, and then later impersonating Tyrael and manipulating Darius into telling him about the world stone.
They all spread their influence, their battle was not with Heaven or the angels, their battle was with all of Sanctuary, the Angels on the other hand, there battle was to stop the demons at all cost, which is why they intervene.
I feel like the Diablo franchise really needs to get back to this premise, no more flashy villains who act like bosses, I wanna open a door and hear FRESH MEAT, and say "Oh f*ck!"