Varus is like Kayn

>He swore to slaughter every Noxian invader, but first he needed to become stronger. He turned to that which he had sacrificed everything to protect. The pit of corruption would consume him wholly, as a flame devours a wick, but its abominable power would burn within him until he was lost.- Varus' lore

If Kayn over powers the weapon, he remains himself, though cursed. Varus could simply have over powered the weapon's urge of dominance therefore remaining human. And it would make sense that darkins choose hosts who are desperate of power, in the price of corruption, as Varus' abilities suggest. and he desperately seeks vengeance.

Also apparently all darkins are Ionian (Varus protecting an Ionian temple which sealed corruption). So that suggests that Zed, in some form sapped powers of a darkin.