Oh word. I remember reading about Diablo story but for some odd reason I thought the angels were bad guys too.
The story is interesting.
It basically goes like this:
There are angels, there are demons, and there are humans.
Humans are part evil in nature.
In order to get rid of the demons, there are some angels that believe in wiping out all evil, even if that includes the humans.
On the other hand there are some angels that don't agree with that strategy and feel the best way is to partner with the humans to get rid of the demons only.
The expansion called "Reaper of Souls" is basically the Grim Reaper taking things into his own hands and attempting to destroy all humans and demons at once.