Originally Posted by
Jastall
Dadgar is quite powerful, but he's "just" a really good Mage. He's not a Guardian level beatstick and doesn't have a track record of easily 1v1ing mighty enemies, for instance Gul'dan make him struggle and kinda bested him in the Tomb of Sargeras audio story. If she's a Void entity near the level of an Old God as the story seems to imply, it would make sense that she's stronger than him.
Besides, Blizzard needs to establish the new villain as a threat, and there's no more tried and true method than having said villain kill X secondary character.