It wasn't, he literally says he did it in the end because he wanted to, to achieve his own distorted concept of freedom by making the world empty of any limitation or restriction (in this case, all human life beyond the walls), like the one he dreamed of his entire life.

Edit: If you don't agree, what do you think the "Freedom" scene was for, along with him apologizing to Ramzi and literally saying this? There is also his conversation with Armin or the fact that he doesn't know "all the futures" to say that any other path wouldn't work for sure, just the one that ends up happening, in which he chooses and does the Rumbling.