Cersei will stop at nothing to stake her claim at the throne. She wants to be Queen, and won't stop until she is.
Cersei currently sits closet to the throne and the power is all hers. If she can just hold off the threat from Daenerys and the Night King the throne will be hers.
With Jaime by her side, she is unstoppable, but she must keep him there or she may not be able to claim the Realm for herself.