The Original Founding Titan Ymir has a very long and baffling saga. From the episode where she would walk into the forest where she acquired her power until the moment of where she dies, it is explained that she is absurdly kind. What a bluff! Anyway, let’s look into her past and how she may have sought “the not so kind” world’s destruction.
Around 2000 years prior, a tribe called Eldian used to conquer villages for power. The leader of Eldians, Fritz would enslave people in order to grow his tribe and hence power. Ymir’s was one of the many villages the Eldians conquered. One day Ymir was set free and sent to the forest where being chased by the Eldian soldiers she hid herself inside a humongous tree, where she would fall into a pond full of mysterious liquid.
In the pool a mysterious-looking creature would bind itself with Ymir hence, forming the first titan befittingly called the Founding titan. King Fritz would use the brute force of the titan to conquer Kingdoms and grow his empire. Despite having such tremendous strength, Ymir shackled her mind to be a slave to the king and his bloodline.
Now 13 years have passed since the origin of Founding titans, however, Ymir is incapable of using her actual abilities as her real powers like titan creation, memory manipulation, etc can only affect her offsprings. On the last day of her life, a mere spear penetrated her, the wound was so garish that she couldn’t transform into her titan form and died. In order to preserve this evolved creation, Fritz made his daughter eat their mother so that they became titan shifters, thus the founding titan was split between her 3 children and this would continue until there were 9 titan shifters in total.
As time passed more subjects of Ymir and hence pure titans were created. Upon her death, Founding Titan Ymir’s spirit was transcended to a deserted realm called the realm of past, filled with sand. As soon as Ymir’s daughters Maria, Rose, and Sheena consumed her spine a path was created connecting the realm and the 3 girls.
This path formation continued as they have more descendants who would consume their spines, and the process continued until it became what it is today. Each path represents a subject of Ymir who is capable of turning into a titan or titan shifter. Every single titan was created by Ymir herself with the help of the sand present in the realm.
As Eren unlocked the full capabilities of the founding titan after coming in contact with his royal blooded brother Zeke Yeager, who possessed the beast titan, in the world of the past he froze Ymir. This rendered her free from the vow she took to enslave the king and his descendants. Here Eren suggests that his attack titan represents freedom, his titan form was so formed that he was unchained from any vow, the blood liners were made to take.
If his attack titan was Ymir’s will to find freedom then all along she has been waiting for Eren to come to free her. Thus, suggesting that she may have sought the world’s destruction with Eren’s help in her own way.