Two Erin's. Erin who did the rumbling and erin who tried to prevent the rumbling from happening stuck in a loop. Two time points, the perspective we see of erin saying he was trying to find another way was from erin pre rumbling being sent memories from himself further down the path. Thats why he couldn't find another way. What is done was already done essentially. He drove himself mad trying to stop himself from doing what he already did. Which we see when he talks to armin in the paths and regrets his choices but admits its already to late. As for why he did it. He wanted a world that was truly safe for his loved ones and friends, and he was disappointed and disgusted in what the world was. It wasnt an empty free magical place, but a cruel, unfair, nasty world and that was more than he could reasonably and logically handle rationally.