They were meant to oversee & protect the afterlives created by the First Ones. They are called the Pantheon of Death.
The Titans were meant to oversee & order the material plane. They are called the Pantheon of Order.
The fact they embody different motifs of death that exist in the material plane is just a chicken/egg situation.
Not to mention the idea Elune & the Winter Queen are somehow siblings or how the Primus hid the Tools of Domination on Azeroth without the Titans knowing suggests the Pantheons did not always exist the way they do now.