So basically what you're saying is time and time again the playable races have rebelled against their past darker origins and that somehow the idea to have corrupted Eredar break away from the Burning Legion in some future Burning Legion expansion is just too far-fetched. That's actually pretty funny.

The Alliance also doesn't have a single religion because it is a political faction with a coalition of mutual military and economic aid. They have do not have unified religion or law that would specifically say they are completely against the use of fel magic and the same applies for The Horde.

Furthermore exceptions are always obviously made just look at the lore behind Warlocks, Death Knights, and Shadow Priests. Those three classes alone have many controversial issues and stigma attached to them and they even contradict other classes' beliefs and practices i.e. Holy Priests, Druids, Paladins, etc. With the flick of a pen everything all of a sudden makes sense in a magical fictional universe.

Arguing against my suggestion in a fictional universe where the completely fictional writing can be taken in totally different directions is just silly.

The fact that you say things like "The Alliance doesn't like fel magic" and don't realize everything you've said about the other races is literally the writers making exceptions is sad.

There is always exceptions, there is always different view points within every faction. A perfect example of this is the Worgens being accepted into the Alliance because of the Night Elves reintroducing them because guess what? The race of Worgens alone was feared by both members of Horde and even Alliance and the idea of them joining a faction was crazy at one point.

I could go on a roll with Warlocks, Death Knights and Shadow Priests even being allowed to practice their magic in either faction or we could talk about several iconic characters. Good and evil is just a point of view. The entire war between the Horde and the Alliance is the perfect example of this.