The thing for me is this. Aggramar was "killed" before the others had a portion of their souls saved and fled to Azeroth. Now I can understand Sargeras trying to preserve and bring Aggramar around to his side since the two were close comrades after fighting the void for so long together.
But 1)How did he gain Eonar's soul to place into the Avatar and 2) why would he try to res her when lorewise the two of them disliked one another?