>>118233705tl;dr, the people on the island are known as "eldians", they're the only race able to turn into titans, and for the longest time were kinda assholes to everyone else, so eventually they revolted, which lead to a group escaping to madagascar, setting up walls made out of tens of millions of 50m titans, and threatening if anyone messed with them, they'd be set loose and stomp every inch of the world flat. Long story short, they messed with them, Eren decided enough was enough, and kickstarted the plan to curbstomp the planet.
