>>12306545The rumbling was about fulfilling his childhood dream of achieving freedom.
Eren didn't really aspire to save his race, he wanted to experience his childhood dream of what could be beyond the wall.
Sadly, when he first left his island, what he found was the same corrupt world which was in his own homeland. He didn't seek for this world and he didn't want to accept it, so he decided to cleanse it in order to achieve his dream, freedom.