The Chosen Ones

If I'm right , the dragons created the human race as a "slave" race.Then a human escaped the slavery and killed one dragon.Then all the other humans escaped , and the dragons were like "Think you can escape me ****?Here I come!".It's just an oppinion .

Would be nice if it was that easy !
No, actually there are more groups. In fact, Aero even let slip that there was a group that wanted to fight the humans and one that didn't but live in peace with everybody and even quite a few more.

Death Knights serve Zylcious.
Xvareon is not the strongest dragon (I'd say that's Yurgimar at the moment).
Death Knights can be fallen Dragon Slayers, but don't have to. I think it's enough that they are or were strong warriors. After all Zylcious choose them by himself and I don't think it's that important to him if his strong servant was a former Dragon Slayer or just another guy.