WARNING: The following contains major spoilers for the Attack on Titan manga, available in English from Kodansha.
Attack on Titan's Grisha Yeager has long been considered one of the worst parents in all of anime, unceremoniously inducted into a hall of fame consisting of such vile and irredeemable characters as Shou Tucker from Fullmetal Alchemist and Gendo Ikari from Neon Genesis Evangelion. However, recent developments in the Attack on Titan manga have completely turned the perception of Grisha around, not just somewhat redeeming him but also painting him in an unexpectedly sympathetic light. Grisha Yeager may not be a good father, but with these recent revelations, he's definitely far from being the worst.
The negative perceptions around Grisha first sprung up when, in a flashback, he was shown to be the one to inject Eren with Titan serum, thus transforming his own son into a monster for the sake of his own vague motivations. Promptly after this, he was eaten by Titan Eren in his mindless condition. Many interpreted this as Grisha receiving karma, but it's since been revealed to be intentional in order to pass on the Founding Titan within him to Eren. The Founding Titan itself was inherited by Grisha after he ate the previous holder of the power, Frieda Reiss, in the process also killing almost every member of her family. Ever since then, Grisha Yeager has been considered a man capable of committing disgusting acts for selfish goals.
The first signs of an altogether different narrative surrounding this character appeared when his previously unclear goals came to light in a flashback detailing his backstory. It's revealed he was born in an internment zone where the rest of his kind, called Eldians, were oppressed by the Marleyan government. At one point in his childhood, he and his sister decided to sneak outside of the internment zone to see a blimp land. Venturing outside led to grave consequences, as Grisha's sister was gruesomely fed to the dogs by the Marleyan guards that caught them.
Traumatized by the death of his sister, Grisha grew up to join a secret Eldian resistance movement vowing to achieve freedom for their race. Grisha eventually married one of the key members of the movement, Dina Fritz, a descendant of the Eldian royal family. They had a son together, Zeke, who Grisha, in his revenge-fueled desires, attempted to indoctrinate with Eldian propaganda. Zeke, being a child under immense stress and pressure, ended up reporting his parents to the Marleyan government, leading to Grisha and Dina being sentenced to roam Paradis forever as mindless Titans. Zeke himself eventually grew up to take up the opposite goals as Grisha, attempting to "euthanize" his own people.
Moments before Grisha could be transformed into a Titan, however, one of the Marleyan soldiers revealed himself to be the Attack Titan, killed the other soldiers, and rescued Grisha from his nightmarish fate. That soldier, Eren Kruger, revealed to be an undercover Eldian resistance fighter, tasked Grisha with the mission of infiltrating Paradis and obtaining the Founding Titan before Marley could. Grisha then consumed Kruger to become his successor.
With a traumatic childhood, an oppressed race, a lost wife, and a son that betrayed him, just one chapter transformed Grisha from a vile character into a sympathetic tragic one. Still, despite this newfound context to Grisha's character, many fans still believed him to be a horrible father for trying to indoctrinate his first son and forcefully involving his second in his mission. However, even this narrative is soon put into question in the most recent chapters of the manga.
In a shocking turn of events, it is revealed that Eren, due to being the Attack Titan, has the ability to influence the minds of his Titan's past holders. Zeke, who believed that Eren, like him, was a victim of their father's indoctrination, quickly found out that Grisha was not just regretful of how he raised Zeke, but was also adamant in not making the same mistake with Eren. Instead, it was Eren who, using his paradoxical time-manipulating powers, convinced Grisha into injecting child Eren with the Titan serum and to kill the Reiss family.
Suddenly Eren is no longer a victim of Grisha's bad parenting, but instead, Grisha is a victim of his son's manipulation. This shocking twist not just changes Grisha into one of the most sympathetic characters in Attack on Titan, but even redeems him of his vilest actions. Those vile actions now being shifted over to his son Eren, who turns out to be the source of the driving force towards freedom that has pushed the holders of this Titan throughout history.