He only controls all of death, that's all. Nothing major. Oh, and each person that dies gives him more power, and makes it more possible for him to enter this world. When that power gets to 100%, he will.
There's a page from Epicurus the Sage where Hades(who has a lisp) describes being able to to just what Nekron's doing here, building an army of the dead which becomes bigger with every one they kill. "When the dead fight the living the living always lose." I should dig up a scan of that.