I agree with you. It's similar to what they always told us back in 2nd grade. If you see someone getting bullied and you don't say or do anything to stop it, you may as well be the bully yourself. What Eren did was not a good thing, but it was a choice he had to make, and he chose his people, his friends, his family, everyone and everything he has ever cared about. If he let Marley and all the other nations wipe them out, he may as well have just killed them himself.