Why did Eren Yaeger's character change?

You have to look at the other half to learn what’s going on in him.

It’s like this:

At the moment when Eren kissed the hand of Historia, he saw the future. He was shocked by what he saw and also knew he could not change the future. Even if he had not seen the future, he would have implemented his intention (the crumbling of the world).

For this he needs to contact Zeke to get to the coordinates to find Ymir Firtz and to give her the order to release the Titans in the walls to the world.

His goal is to put the rest of humanity into a disointed state, so that people on the island of Paradise can build/continue, and their remaining friends can live a better, calmer life by standing there as heroes at the same time, because they have finally stopped him through his killing.

He is not really evil, he suffered very much and knew he will die, but did not want to die and be with Mikasa as he will reveal tears himself when he talks to Armin. Unfortunately, there was no other way out for him.

Past, present and future existed for him at the same time.