So I've always had a little trouble following the diablo storyline (partly because I only played the 2nd one), but after looking up summaries online it makes a lot more sense to me. However I do still have 1 big question. Why do you only fight demons in sanctuary?
-Everyone says that neither angels nor demons are inherently good/evil, theyre just two different sides to the war
-Both sides found the entrance to Sanctuary
So why are demons the only ones trying to kill the Nephalem (the humans are what the Nephalem have devolved into, right?)?
Short, less imaginative answer: The whole Angels not being necessarily good/on our side is relatively new lore, only really brought up by the Sin War Trilogy. Up until then, including the games Diablo I and Diablo II, as far as I know, Angels were never indicated to be anything other than benevolent/on our side. So, they weren't a threat to us because they hadn't been described as that yet.
More involved answer: Humanity isn't necessarily good or evil. We get a choice in the matter, which makes us capable of being better than angels and worse than demons. So, Angels have a reason to be concerned about us being a bigger problem than Demons, and we do have room to look down on Angels, dependent on our choices.
to clear up some mis-conceptions, it should be noted that Angels (particularly those of the Angiris council) are embodiments of particular virtues: Justice, Hope, Valor, Wisdom, and Fate. As of yet, only one of those, Valor (Imperius), was particularly hostile to us. Imperius is the most warlike (his point is to engage in glorious combat against evil), and his impulse is to immediately fight and kill anything slightly demonic, consequences be damned.
That said, Imperius was outvoted, 3-1. Auriel, Angel of Hope, liked us right way, taking what seemed to be a maternal liking to us. I suppose you could draw some ties between maternal instinct and hope. What is more hopeful than a mother?
Ithereal, Angel of Fate, who finds us confusing, also voted to keep us around, seeing great potential for good in us. Actually, he thought we had more potential for good than Angels, which actually makes sense. Free will is the key. How good can you be if you don't have a choice. In the words of Ann(e) Shirley: "I'd rather have a man who could be evil, but wouldn't be." In other words, to choose righteousness is better than to be inherrently righteous.
Tyrael, Justice, while initially was perfectly fine with the idea of killing us all, saw the good in us, mostly in Uldyssian's actions, and has seen fit to fully ally with Humans. He does see our potential, and is impressed with our ability to choose. Granted, he pulls a lot of strings, using us as chess pieces against the Evils, but also does care somewhat for at least some of his chess pieces. He swore to look after the descendents of the Horadrim.
Finally, Malthael, Archangel of Wisdom abstained, not really caring. As I understand it, he used to be much more sympathetic, before the Worldstone was stolen. He then got into kind of a funk and started looking like the Grim Reaper. No one's seen him since it was destroyed.
So, in recap, 1 archangel is willing to go on a crusade/inquisition and kill us, seeing us as impure. 1 angel doesn't care, seeming to be going through a bit of an identity crisis. The others like us, have high hopes for us, and think we can be better than they are. One of which is our ally. Not exactly a picture of angels cleansing sanctuary of human life.
That said, I think there is some room for a bit more variety. Perhaps, with the worldstone blasted off the face of sanctuary, Malthael will completely embrace the Death Portfolio. Neither he nor Trag'oul, the Necromancer's Draconic Patron, have been seen since that fateful event. With the Lord of Destruction so intimately involved in that, perhaps will see a nice death crusade coming. That and I think Imperius has a part to play.
What would really do it for me though is a Demon, perhaps not Lord of Hell level, but a reasonably powerful Demon on level with say Lillith or Lucion, who is on humanity's side, but certainly not a "good-guy." One who doesn't mind siding against the Lords of Hell. Maybe Lillith could do it.