I think what people are saying is that if there's really to be 4th spec then DH will fit in Warlock the most which is true. That doesn't mean DH is Warlock.Exactly!
A character can become any class they want, even if they were previously another class that has little to do with that new class, provided the excuse for such is well-made... but that has nothing to do with combining classes or making one the subclass of another. Demon hunters and warlocks are similar. This is fact. They both wield fel magic. However they have different means of gaining and wielding these powers as well as different general intentions. Warlocks don't fully represent demon hunters, nor do demon hunters fully represent warlocks. There are enough differences between them that they are not the same class.
Illidan never really made a pact. He was "rewarded" with his power for swearing to serve Sargeras and helping him out in some way (I forget how, exactly).
As well, warlocks are mages who sought more power and turned to the fel arts for such. That is a fact. That doesn't mean they have to be bad guys. It just means that they ultimately desired power. They are viewed as bad guys by society because they broke the taboo of magi and turned to fel magic, not because they sought more power.
Last but not least, Benedictus was still a priest and still adhered to its general archetype, namely the archetype of a shadow priest. It's the antithesis of a Light-wielding priest, but it's still a priest archetype.
Illidan was hungry for power, true. However his ultimate intentions were to ensure that the Legion would never be able to take Azeroth, which is the commonality of demon hunters; to deny the Legion Azeroth and its denizens.