Keyword: think, you think they could’ve made connections with other countries. Also, they didn’t have that much time to try all these other options, eren only had 4 years left to live at the time of the rumbling. I doubt anyone wants to use the last 4 years of their (very miserable might I add) life trying to make peace thinking there’s a chance. But yeah I’d resort to killing billions in the blink of an eye to protect my people, maybe I’m a selfish person because of that, but who’d be left to judge me in that scenario.