I'm not trying to understand this poorly written plot since I'm here only for the fanservice, harem and power fantasy.
But either this monster girl ploted the attack against Egil and now offers him a solution so she can infiltrate his Kingdom and destroy it from within or this girl has a bigger threat that she can't beat alone, thus forced her to join forces with her rival/enemy Egil so she can evacuate the monsters she loves. 50/50 for both scenarios.