Then Rhea's actions are not preemtive, they're reactive. It wasn't just a heist, Edelgard brought actual soldiers, with the intention to kill if needed. Also, this is where she outs herself as the Flame Emperor, also outing herself as being in cahoots with the people behind stuff like the Remire Tragedy. Lastly, in Rhea's eyes, she's gonna do the very thing Nemesis did, and it's a personal matter since it's the remains of her people. So just because Edelgard failed means she gets to go unpunished? Rhea certainly doesn't think so.