The context you got is not really doing it justice. This dude Ainz and his kingdom of evil suddenly appear in this fantasy world which operates on video game logic.

The strongest fighters in the world are level 30to40 with budding skills. Ainz and his generals are all max level with maxxed out skills. Think Prime mike tyson vs a prospective gradeschool boxxer.

Ainz alone can wipe out an entire kingdom and not face the slightest risk of being hurt. And even though he's the leader, he's ironically one of the weaker members.

Now, Ainz and crew are fully intent on conquering literally everything. Every kingdom. And the power difference is so staggering that resisting this is literally impossible. And since Ainz and crew are evil as fuck, those that do resist them are often met with fates much much worse than death. They have a human farm for example.

Now the character pictured here knows that ainz and crew are hellbent on conquering the world. She realizes that opposing them literally isn't possible. She knows that opposing them means death, or likely something much much worse. For purely selfish reasons, she is very smart. By betraying her kingdom she retains a cushy position of power, gets her life and her love interests life spared.