Attack On Titan: Why Did Annie Betray Humanity? Here's The Heartbreaking Reason
While "Attack on Titan" starts out with a pretty clear emphasis on which side is good and which is evil, as the series goes on, the waters become increasingly muddied. Case in point, the titan shifters who attack Paradis are originally thought to be the villains of the series. This all changes, though, as we learn of their tragic backstories and the ways that their own society placed them in this position in the first place.
Annie Leonheart's (Yu Shimamura-Lauren Landa) case is especially tragic. Having grown up in Marley among the fenced-in ghettos of the Eldian people, Annie's only chance at a better life was to become a Warrior. Her father abused her heavily as a child in hopes of helping her attain the top rank and join the Marleyan forces as a titan shifter. The side effect of this is that Annie grows cold and despondent.
In this way, Annie isn't so much a traitor to humanity in "Attack on Titan" but is more of a traitor to her friends in the walled cities of Paradis. Annie, Reiner (Yoshimasa Hosoya-Rob McCollum), and Bertholdt (Tomohisa Hashizume-David Matranga) all volunteered for the Warrior program before they came to Paradis, but the way they pretended to be from the area led to the Scouts branding them as traitors.
Like many characters in the series, Annie was made into this
Being that the story of "Attack on Titan" takes place on an island, a lot of emphasis is placed on the ocean and what might be on the other side of it. In a clear allusion to what's to come, at the tail end of Season 3, Eren Jaeger (Yuki Kaji-Bryce Papenbrook) points across the ocean to Marley and says, "If we kill all of our enemies over there, will we finally be free?"
This is the ultimate tragedy of "Attack on Titan," as the two sides eventually come to realize that they're both victims of a society that punishes them for being born. They're essentially two oppressed minority groups being forced to battle and kill one another for the sake of those who they serve.
This is why, despite Annie killing countless members of the Scout Corps in Season 1 of "Attack on Titan," Mikasa (Yui Ishikawa-Trina Nishimura), Armin (Marina Inoue-Jessie James Grelle), and the others still agree to ally with her and the other Marleyan titan shifters at the end of the series. They finally see that their years of conflict have created a monstrous force and that it's all of their responsibility to stop The Rumbling at any cost.