Warning: Contains spoilers for the ending of Attack on Titan's manga and anime!
The Attack on Titan manga was one of the most successful of the past decade, thanks to its mix of horror, action, mystery, and socio-political criticism. The ending was a bit controversial - as is often the case for beloved works of fiction - as not all fans appreciated the treatment reserved for the protagonist, Eren Yeager. However, a good look at the final sequence makes it even more depressing.
The final arc of Attack on Titan dealt with Eren acquiring the power of the Founding Titan and using it to bring forward "the Rumbling", a cataclysmic attack on the world outside Paradis Island. Hundreds of thousands of Colossal Titans, previously encased in the three huge walls of Paradis, walked the earth to punish the enemies of the Eldians. In the four days that it lasted, the Rumbling killed eighty percent of the world population, while Eren led his titans on a rampage. He was stopped before he could accomplish the complete annihilation of the non-Eldians thanks to the effort of his closest friends, especially his childhood love, Mikasa. In the final chapter of the manga, however, the true motivation behind Eren's genocidal actions were revealed.
Eren originally inherited the Attack Titan and its unique power, granting its owner memories of the future as well as of the past. With this knowledge, Eren knew that the only way to grant a future to his friends, and the survival of Paradis Island, was to become the worst villain in the history of the world so that his friends could kill him and become heroes, thus changing the perception of the whole Eldian people in the eyes of the world. Also, Eren had come into contact with Ymir, the first titan, and after learning of her tragic story realized what was the only way to make the power of the titans disappear forever. Ymir was a slave to an ancient Eldian king; she accidentally came into contact with a mysterious creature whose only purpose is survival and multiplication. She acquired the titan power, but despite that was still bound to King Fritz, who abused her and her powers, building the Eldian Empire. After she was killed, Ymir became the source of the power of all her descendants, but she still obeyed the orders of the royal family, delivered through the Founding Titan, because she was actually still in love with King Fritz.
When Mikasa finally Kills Eren, despite being in love with him, this is what Ymir was waiting for. Metaphorically, it kills the love that Ymir had borne for 2,000 years for her abuser, who only wanted to exploit her power. Eren saw all this and acted accordingly so that Ymir would finally erase the titan power. Fans were not too pleased with this revelation, as it seemed to redeem Eren after his surprising but convincing transformation into a villain. However, the manga actually ends on a much more depressing note. Many years into the future, Eren's efforts have not stopped mankind from hating each other, as another war lays waste to the land. The tree under which Mikasa buried Eren's head has grown to enormous size. A wandering boy happens near the base of the tree, which looks exactly like the one where Ymir found the mysterious creature millennia before.
The very last panel of the manga is ominous because it's hinting at the fact that history is doomed to repeat itself: the kid will find Eren's head and become the new first titan, restarting the cycle of hatred and abuse. If this is really the final message left by Attack on Titan, it is much more depressing than fans realized.