You might be surprised but Eren is not the first name that comes to mind.
Attack on Titan (Shingeki no Kyojin) has always been famous for not portraying things in black and white; the series constantly tries to give depth to its characters and make you sympathize with them even if they used to awaken uncontrollable hatred in you. The Marley arc alone is enough to turn your view on the world of Attack on Titan upside down and question everything your beloved characters have been fighting for.
Can we really say who the true villain is in a story this gray?
If you're here for the short answer then here it is: no, we can't. Hajime Isayama (the mangaka of Attack on Titan) endowed his characters with various traits and personalities, which make them so relatable, that you are sure to find a piece of yourself at least in one of them.
The author tries to erase the line between good and evil and go beyond these two concepts; he shows characters' motivations and lets the reader and viewer decide for themselves.
And we're going to talk about some of these decisions.
We are not going to mention such obvious villain choices as Eren, Zeke or Annie because we're sure you've seen people arguing about these characters everywhere you go. So let's look at some of the less straightforward candidates.
One of the first characters fans mention when it comes to the topic of villains in Attack on Titan is king Fritz. He is the one who tortured and used Ymir for his wars. He impregnated her and made her give birth to three daughters. Moreover, he forced the daughters to eat their own mother in order to preserve the founder's powers. Even though we might say that the story of titans started with Ymir, the king played a significant role in it. Fritz's cruelty and methods led to the birth of the curse and horrible events.
Next person on our list is the guard who fed Grisha's sister to the dogs. Fans say that he was so hideous that they were actually happy to see his terrible fate. He took pleasure in turning Eldians into titans and watching them suffer, and the audience cannot forgive him for that.
Single characters are not the only ones who can be considered villains in Attack on Titan.
Some fans mention the Marley government and royal families when speaking about the evil in the series. Royal families knew about the curse of titans and possessed more intelligence about all the events, but still decided to keep everything a secret.
And the Marley government was so busy trying to enslave the rest of the world, that they didn't notice how everything got out of control. Them treating Eldians as dangerous beasts and keeping them locked down made the situation even worse.