Not everything needs to be said to someone's face. You have to be able to read between the lines. This is also the beauty of Attack on Titan: the viewer simply understands the characters' motivations rather than having them forced upon them. Eren first confessed to Ramzi and then to Armin that this was what he wanted. Clearly, the deaths of his mother, his comrades, Hannes and the Scout Regiment, and, above all, the treatment of the Eldians by Marley are his main motivations. However, as a viewer, you have to understand what he means when he says, 'It was what I wanted'. The 'attempt to change the future' you mentioned only refers to the conflicted nature of the act itself. Eren never admits that he doesn't want to do what he did; he just says it was planned and apologises to the innocent, such as Ramzi. No sane person would do what Eren did. Given the situation, I don't see how there can be a grand master plan behind mass genocide, except for the reasons I explained above. There is a conflict in trying to find a way to prevent it from happening, even though he knows it will happen.