She wanted to take over the Slime Kingdom and turn it into a military force to take over all of Ooo. In order to get control of the kingdom from her sister she had to get married, but by Slime People standards she was so attractive no man could work up the nerve to approach her. Therefore, she made a fake husband named Guillermo and was about to take over the kingdom until Slime Princess convinced Finn to pretend to be her husband and they eventually found out the truth about Guillermo, thus disqualifying her challenge for the kingdom's rule.