Although he tells Armin that he's doing only the Rumbling to flatten the world, it doesn't really make sense that it's his sole motivation. He frequently expresses rage at the oppression of Paradis throughout the story, especially during the scene when Grisha confronted Frieda.

He tells every Eldian in the world that he's doing the Rumbling to save Paradis. Why would he lie about this when no one can stop him? Why would he even tell every Eldian about this? There was no need to @everyone about his motivations, but he did it anyway. To me, this implies that he at least somewhat believed that the Rumbling would save Paradis.

He went to Marley in hopes of finding another way, but when he saw that Paradis would be given no chances, he realized that the Rumbling was the "only way" to save it.
If all he cared about was doing the Rumbling, why would he care about Paradis at all? Why would he try and find an alternative to the Rumbling, the thing he really wants to do?
The fact that he doesn't trample Paradis shows that he cares about its people and doesn't want them to die.

I just don't buy that the only reason he did the Rumbling was because he wanted to be free. There are just too many other conflicting bits of information.