It was a revenge plan for almost 12 years. From the day Maine lost her family, through the days of her dreadful training, through the years of her servitude as a slave, through the decade of her life as a Rein Empire's Assassin, until the day that she finally decided to confront her Hero that she cursed and the child she despite.
Maine couldn't infiltrate the Green Dale Town at the thought that Leaena would recognize her. That's why, instead of directly confronting Leaena and Felin, Maine chooses to infiltrate the Smilodon Kingdom, looking for a high-status individual she will be able to manipulate to bring pain and suffering to Leaena and her new family.
With the lies Maine fed to Duke Genetta about the plan to kidnap Felin and blackmail Leaena into marrying the Duke, Maine was able to gather enough money and support to move on with her plan of revenge.