If we want to blame one family for the destruction of over 80% of the world, then it has to be the Yeager family. In the world of Attack On Titan, it is not an exaggeration to say that The Eldian painted much of the history of the world with violence, blood, and war. Throughout all of that bloodbath and conquest, one family stood alone as the conquerors through the power of their Titans. They are the Fritz family, who later on change their name into the Reiss family.
However, if one family was responsible for shaping the world as we know it today, another family is responsible for destroying over 80% of the world. They are the Yeager family. And if The Fritz needs a massive empire, the entire Titans, and generations of descendants in order to influence the history of the world, The Yeager only needs three men to burn it all to the ground. And this is the dark legacy of the Yeager family. Grisha was born and raised in Marley. And just like any other Eldian in Marley, he lives in the Internment Zone in Liberio and faces verbal and physical abuse on a daily basis. The first major event in his life is when his younger sister, Faye Yeager, is killed just because they went out of the zone to see an airship. This event awakened the rage and hatred in his heart towards the people of Marley.
Young Grisha Yeager grew up to be a doctor. During this period in his life, there are two major events that change who he is as a person.
The first one is when he meets Dina Fritz, a girl with a royal bloodline who later on becomes his wife. The second one is his activity as an avid member of Eldian Restorationist, an extremist who not only wishes to restore the Eldians glory days, but also to exact revenge on the cruel Marleyan.
Grisha and Dina has a son named Zeke Yeager. However, rather than giving their son the love and attention that he deserves, they shove their ideology on him and force him to join the Marleyan military and essentially become a spy for them. This proves to be a much too heavy burden for the 7-year-old Zeke, and he ends up reporting his parents' forbidden activities to the Marleyan military instead. Not long after that, Grisha and Dina are brought to the Paradis Island where Dina is turned into a Titan while Grisha inherit the power of the Attack Titan.
Within the walls of the Paradis Island, Grisha married a woman named Carla and have another son named Eren. He keeps working as a doctor, but in secret, he tries to find the location of the Reiss family. When he finally found it, he massacred them, leaving only one man behind. Not only that, when the walls of the Paradis Island is breached, Grisha gives Eren the Titan serum and let Eren eat him, which effectively pass on the power of the Attack Titan to Eren. Zeke never felt the love that he needed from his parents. As a young boy, he was forced to do something that he hated for the sake of his parents' dream, and still his father would berate him to no end even though he always did his best for them. That is why Zeke didn't really feel bad when he reported his parents to the Marleyan. As a matter of fact, it was such a liberating moment for him.
Nevertheless, Zeke still inherits his father's will to free the Eldian people from their suffering. However, he arrives at an entirely different solution. Rather than helping his people to rise above and defeat the Marley, he plans to make every subject of Ymir sterile through the power of the Founding Titan. This means every Eldian will lose their ability to reproduce and after around 100 years, there will be no Eldian left. To Zeke, having never been born is better than having to live in this everlasting hell. In order to execute his plan, he needs the power of the Founding Titan in Paradis Island. So he becomes the leader of the Warrior unit within the Marleyan army, and lead the invasion of the Paradis along with his fellow Titans. Zeke and his team, such as Reiner, Annie, and Bertholdt, would kill hundreds, if not thousands, of innocent people in Paradis island, and that is the price that he is willing to pay in order to achieve his goal. Now we come to the one and only Eren Yeager. Compared to his father and half-brother, Eren led a relatively normal childhood with his best friends Armin and Mikasa. But everything changed when the Colossal Titan attacked and destroyed the gate of the Wall Maria in the Shiganshina district. He's been fighting as the Attack Titan and part of the Survey corps ever since.
There are two major events that slowly but surely push Eren into the edge of madness. The first one is the truth about the Titan and the outside world that he finds out from his father's journal. The second one is when he touches Historia's hand that allows him to unlock the memory of the previous Attack Titan, his father. The knowledge that he receives from both events planted a very sinister idea in his mind.
In order to attain true freedom, both for him and the rest of the Eldian people, he needs to initiate the Rumbling.
After becoming a terrorist, killing thousands of innocent people in Liberio, and betraying everybody that are close to him, he finally attains the full power of Ymir, and started the Rumbling. When he is finally stopped and killed, Eren has killed around 80% of the human population and erased the power of Titan from the Eldian people.
Had Grisha given up on his extremist ideals and choose to live happily with his family. Had Zeke decided not to urge the warrior unit to infiltrate Paradis island. Had Eren decided to abandon his father's journal and be content with how they manage to push back the invaders, all of this bloodshed wouldn't have happened. And yet these three men chose to pursue their goal, and the inevitable global genocide finally happens. That is the dark legacy of the Yeager family.
