WARNING: The following contains spoilers for Attack on Titan Season 4, Episode 28, "The Dawn of Humanity," now streaming on Funimation, Hulu and Crunchyroll.
The finale of Attack on Titan, The Final Season: Part Two harkened back to the series' beginning in the darkest of ways. When a young Bertholdt Hoover used his Colossal Titan to destroy the gate of Shiganshina so that Titans could get in, Eren Jaeger's mother was eaten right in front of him. After evacuating, a tearful and rage-filled Eren declared he was going to kill every single Titan in the world. Fast forward to Season 4, Episode 28, "The Dawn of Humanity" and Eren is still saying "kill them all" -- but now he's saying it while using Titans to wipe out humanity.
When a young Eren Jaeger declared he would rid the world of Titans himself, it appeared to be little more than wishful thinking. At the time, even Mindless Titans were unstoppable as humanity within the Walls knew next to nothing about their greatest enemy and didn't even know about the existence of their actual enemy. At the start of Attack on Titan, there was nothing Eren hated more than Titans, and who could've guessed that becoming one himself would still be considered "simpler times" in retrospect. As things spun out of control on Paradis Island with the reveal of several spies from beyond the Walls who could become Titans, even Eren couldn't keep track of who his enemy was anymore.
After the defeat of Zeke Jaeger, Reiner Braun and Bertholdt Hoover in Shiganshina when humanity within the Walls reclaimed their lost territory, Eren finally accessed his father's basement and learned the truth of the world. The nation of Marley became Eren's new enemy when he learned that they'd been the ones turning people into Titans and sending them to Paradis Island to wreak havoc on the unsuspecting population. The revelation that humanity was not only thriving beyond the Walls but also trying to kill them, was a lot for Eren to take in all at once and understandably warped his identity on a fundamental level.
Eren's heart was broken when the citizens of Paradis Island finally defeated Titans and reclaimed their territory, only to learn that even more enemies awaited them across the very sea that Eren and Armin had longed to visit. Eren had always felt like he was trapped in a cage while growing up inside the Walls, with the threat of a Titan invasion looming over every moment. The fact that the outside world he'd grown up wanting to visit was just another cage surrounded by enemies officially shifted his pathological hatred from Titans to everyone outside the Walls.
The war between Marley and Paradis Island is long and complicated, but Marleyans aren't the only people who despise Eldians. Just about the only thing that the entire outside world of Attack on Titan agrees on is that Eldians are Island Devils who should be eliminated. Now that Eren has obtained the Founding Titan, he has chosen to save Paradis Island by destroying every inch of the world beyond its shores. "The Dawn of Humanity" saw Eren become the very thing that drove him to this point in the first place as his Founding Titan descended upon a helpless city, slaughtering countless innocent civilians who don't even understand what's happening or why.
Eren's original "kill them all" was a reasonable emotion to feel toward the clear-cut enemy of the series, Titans. Now the world of Attack on Titan has become infinitely more complex and Eren's latest "kill them all," is understandable, but not sympathetic. His journey from protagonist to antagonist has been a poignant, emotional character evolution of an angry boy growing into a vengeful man. Things are looking pretty grim -- and are likely to get even worse -- once Attack on Titan's Final Season returns for Part Three in 2023 as Eren Jaeger's war against the world comes to a head.