I believe the antagonist isn't always the obvious villain. Take 'Death Note' for example—Light Yagami starts as the protagonist, but his god complex and ruthless actions make him the antagonist by the end. Similarly, in 'Code Geass', Lelouch's ambition blurs the line between hero and villain.
In 'My Hero Academia', Shigaraki Tomura is a classic antagonist with his chaotic ideals, but his backstory adds layers to his villainy. Meanwhile, 'Attack on Titan' flips the script with Eren Yeager becoming the antagonist in later arcs, challenging the audience's loyalty. Antagonists like these aren’t just obstacles; they’re mirrors reflecting the flaws and conflicts within the protagonists and the world they inhabit.