WARNING: The following contains spoilers for multiple episodes of Attack on Titan.
Attack on Titan has a huge ensemble of characters, and many of them would make compelling protagonists. Whether that's Historia, the illegitimate daughter of the king, Levi, a street kid that became humanity's strongest soldier or Mikasa, the girl with a mysterious bloodline that allowed her to become a force of nature - any of these characters and others could be the dynamic hero. But the protagonist is Eren Yeager. A boy blinded by revenge and a failure that can't stand on his own.
In every season, Eren has taken on the role of the "damsel in distress." Instead of using his own strength, wits or Titan abilities to get by, it's always up to his friends to rescue and protect him. This has lead to a lack of self worth, a resigned outlook and a suicidal nature that were even addressed in his most recent kidnapping.
When he and Historia are brought to Rod Reiss, the true king of Eldia, Rod tries to convince them that the only way to save humanity is to have Historia turn into a Titan and eat Eren, who submits to the idea. Historia, on the other hand, sees through Rod's manipulation and saves Eren, calling him out for wanting to throw away his life. Even when the Scouts come to their aid, Jean and Connie mock Eren for acting like a "tragic hero" in his darkest moment.
And they're right, to an extent. Though his character is tragic, Eren is not a hero - and that's the beauty of it. While a traditional anime protagonist is the hero who saves the day, Eren starts out as a civilian who dreams of being a soldier, but when confronted with war the realization that he isn't a savior hits hard.
His mom, Levi Squad and Hannes are the main characters Eren blames himself for being unable to save. With each of their deaths, his mentality and self worth decline, showing how cruel war can be on a child. Eren's failures and even his - comparatively little - victories are contrasted against the heroic deeds of other characters. In Season 1, despite Eren damaging the Female Titan, it's Mikasa who prevents Annie's escape. In Season 3, Historia kills Rod's titan, Levi eliminates Zeke's army and Armin risks his life to give Eren the chance to incapacitate the Colossal Titan - but Eren does very little on his own.
While Eren helps in many of these attacks, he's never the hero. Characters originally seen as weak, like Historia and Armin, and characters who better grasp the weight of sacrifice, like Mikasa and Levi, are able to shine on their own. Eren may be a soldier, but he still has a civilian outlook. Him becoming disillusioned with what it means to be a hero and learning that he is not special are ongoing themes in the show.
When Eren gets closure about who his father is and why he has Titan abilities, he realizes he's not special - he's just the son of someone who was. But, it's not heroics that should determine someone's worth, it's the fact that they are born into this world at all, according to Carla, his mother. Eren, like everyone else, should be able to live a free life, but is unable to because blind hatred has stripped away their basic human rights.
While the surface conflict of Attack on Titan is about defeating the enemy, the real story is about the tolls trauma, bigotry and war take on innocent lives. Eren is not supposed to be a great protagonist because he is just another victim of a world that consistently takes away his freedom. From day one, Eren has no liberty: he's trapped inside Eldia's walls and behind enemy lines, used as a weapon or turned into a monster. But, Eren's a bad hero, he's the right protagonist for Attack on Titan. Seeing his story unfold solidifies how important independence is because, without it, mentality and morality decline.
Eren is often blinded by vengeance, waxing psychotic as he fantasizes about how he is going to murder those who have hurt him. It's not an admirable trait, and no tragic backstory can excuse immoral actions that perpetuate the very cycle of subjugation he's trapped in. That being said, Eren does have shining moments as a hero, and these are often when he is acting of his own volition. When he chooses to risk his life to save Armin in Season 1, defend Mikasa from the Smiling Titan in Season 2 and consume a vile of Titan serum to protect the Scouts in Season 3, those moments are when he is truly a hero.
With Season 4 on the horizon, Eren and crew now know there is an outside world. The physical walls are no longer there, but Eren understands that freedom is still out of his reach, so it's up to him to decide how he wants to reclaim it - by continuing the cycle of violence or breaking it. Eren's existence lays the foundation for Attack On Titan's moral center and the choices he makes going forward will determine what kind of world the Scouts will be able to forge. It's the burden of a hero, just not a traditional one.