The ending was perfect. You guys just didn't understand the story.

Eren saw a false vision to trick him into thinking he had to do the rumbling, and was used as a conduit for Ymir to gain control to get revenge on humanity. Eren went on auto pilot shortly after his speech and was in a state of delusion and falling out of consciousness throughout which is why he ceased to be a character for a majority of the final arc until the final chapter. Which is why when he woke up from his slumber he started incoherently trying to justify his action until realizing he had just been hoodwinked and had been tricks into doing the rumbling by Ymir, and in defeat admitted he was an idiot.

Armin being a nice guy tried to help Eren save face, but Eren was a guy who got a huge God complex and was the prime candidate for Ymir to use as a puppet. Armin tried to string together some sentences like thank you for becoming a mass murder for our sake to make his friend feel better about being tricked, but couldn't help but add a bit of snark due to being baffled by the idiocy of his friend. And quickly added we'll try to fix your mistake. It is also why Armin was able to convince people like Zeke and Kruger to his side, since all he had to do was reveal that they were following the plans of an idiot who wasn't in charge in the first place and wasn't even aware of what was going on. Some of you guys like to poke fun about the leaf, but it was Armin's way of saying who you are following is so dumb you might as well be following a leaf.

Also, it was why Mikasa was able to continue to obsess over Eren, since she was able to tell herself that wasn't Eren who was causing the murdering of millions maybe billions of people. It was Ymir controlling my sweet stupid Eren into freeing her to be able to exact her revenge, and Eren was nothing but a dumb empty headed pawn sleeping throughout. Just a battery. All foam and no beer.

As for Historia she did say she is an enemy of humanity so was totally cool with genocide and was acting when she made it seem like she was shocked by Eren's plan. Which is why she didn't bother to stop him, because she actually delighted in the idea of genocide. Didn't really care what happened, since she's delights in unnecessary chaos and probably was the instigator driving Eren towards a twist savior complex of believing he had to murder everyone which is why she was the only one of his friends told about the plan. She was probably an agent of Ymir. Her leading the fascists pro genociders is a reveal of her true intentions with the baby having been a ruse to trick people into thinking she was just an unassuming housewife.

So you see. It all makes sense once you realize Eren was a pawn. Not the master mind, but more like the gullible wannabe Hollywood actor who ends up getting taken advantage of by their agent and losing all their money. Or think those corrupted by the ring in Lord of the Rings, so Eren is basically Smeagol. Ymir is the true MC of the series.

You guys failed to understand the story because you approached it thinking Eren was meant to be taken seriously when the final chapter clearly laid out how delusional and confused he was showing he clearly was not the one in charge. Once you realize that the ending makes sense. And because so many of you didn't understand the story the anime attempted to get that point across by having Eren say he is an idiot, and still you guys cling to the idea that Eren was coherent and wasn't being misled. The ending is not flawed. Eren was always meant to be an idiot and to show how idiots can rally other idiots into really not well thought out plans. If Eren hadn't been stopped or in other words Ymir's battery disconnected then Paradis would have been destroyed too by puppet Eren while he remained oblivious as Ymir showed him a false reality.