The episode ends on a fairly happy note by "Attack on Titan" standards, but the post-credits scene changes everything. As the years go by, Mikasa continues visiting Eren's grave and eventually she's accompanied by what appears to be her husband and their child. We then see that Mikasa grows old and passes away, buried with the red scarf and the flowers that were on Eren's grave.
Time keeps racing by, and as the tree grows, so does the city behind it. We see cars and airplanes zoom by; skyscrapers climb higher into the sky, and eventually the city is ravaged by war. Missiles destroy the city and nature reclaims the area, with the tree towering over the landscape. A boy and a dog explore the area and notice the tree. He's wearing a scarf similar to Mikasa's, suggesting that he's her descendant, and starts approaching a hole in the base of the tree. The scene is remarkably similar to when the Founder Ymir found the power of the Titans, implying that history repeats itself.
All of the war and destruction in the post-credits scene suggests that violence is part of the human condition — it was never connected to specifics like Titans or racism against Eldians. Despite Eren's attempt to level the world and create a future where everyone is united, people continue finding reasons to fight against one another.
