So Alleria doesn't know that Lor'themar and Liadrin don't live in Stormwind?4. Alleria has plans to conquer Stormwind and kill Lorthemar, Ladrin, and the Blood Elves.
Alleria is not a bloodthirsty murderer, she has no intention of ruling over a graveyard. That is why she wants to bo BRING SILVERMOON INTO THE ALLIANCE, not DESTROY IT.