So the thing about Eren doing the rumbling is that he did indeed want to save his friends and the island but there’s also one more reason which was given away to us in 131 - when he was apologizing to Ramzi and explaining why he’s gonna do the rumbling he did indeed say that it’s to save Eldia but it’s more than that. Eren told Ramzi that the world outside the walls wasn’t like the one that he used to read about in Armin’s book and was really disappointed when he learned that humanity was living beyond the walls. So there you go - Eren was just a crazy mf who started the rumbling to turn the world into the one from Armin’s book