What is important to remember is that, unlike every other mortal up to that point, Alleria was lucky enough to meet Locus-Walker, an ancient being who studied and wielded the Void at an exceptional level. We see in Mac'aree through a questline that Locus-Walker was training Alleria to wield the Void as a weapon. And at the culmination of the story, when Alleria absorbs the Void Naaru at the source of Mac'aree, the Locus-Walker tells her that she has become the weapon itself.
Alleria is very cunning, resourceful, and dedicated to the goal, but we must not underestimate the importance of Locus-Walker; as a teacher, mentor, and above all else, a genuine friend to Alleria. I think Locus-Walker is one of the most ominous, but also wholesome, characters in WoW. He selflessly helped Alleria control her powers (powers she accidentally took while on a Legion world) and he is a true friend to her. To the point that he is now training Alleria's disciples, the Ren'dorei.