It takes real talent to make a hero a villain and make it have total sense. And Attack on Titan manages to do exactly that.
I really like they made Eren an asshole. It shows that both sides aren’t really in the right. You could make an argument that Attack on Titan has become a satire of how war is never a good thing no matter what motivations you may have. Not a comedic satire like Dr. Strangelove, but it still feels very much like a critique of how war can change people for the worst, such as with what we see with Gabi.
The moral of Attack on Titan is basically war is bullshit. It can turn good people and make them do horrible stuff.