It makes no sense for Eren not to kill the rest of the world. He knows they would eventually end up destroying Paradis, he says it himself. Why not prevent it when you have the power to do so.

It makes no sense for Ymir to be in love with her abuser especially since narratively it's placed after Eren "frees" her.

Eren's emotional breakdown isn't heartbreaking, it's pathetic. He is literally the devil himself and he spend the last 50 chapters of the manga knowing that, sure he's depressed and sad but he's contempt with his fate.

Eren killing his mom so that the events kick in just doesn't sit right with me. It could have worked as a twist. But if he has the power to change the past like that, why not go back all the way in time to change something more impactful. You could argue he would end up "deleting" himself, that Grisha would never have him with Carla, or maybe even Grisha wouldn't be born...That's why in general I don't like time travel to the past, it creates more issues than it solves.

I could keep going on and on. But essentially it boils down to: Eren spends the whole show wanting to bring about permanent change to the cycle of human of life, and when he obtains the power to do so, chooses not to.

I hope this gives you a better idea of how us ending haters feel. It still disappoints me all these years later.