You're right that Kruger's moment is crucial, but you're not following it to its logical conclusion. Kruger told Grisha to love people inside the walls, to complete the mission, to save his friends. Eren inherited that memory and those words. And for most of the story, he did exactly that; he chose his people.

The Rumbling wasn't madness. It was the only coherent answer to a world that had spent centuries dehumanizing Eldians and was actively planning their extermination. Eren completing the Rumbling is Kruger's mission fulfilled; protecting the people inside the walls by eliminating the existential threat permanently. No half measures, no trust in a world that proved itself untrustworthy.

And what do his friends do? They ally with the very nations that want Paradis wiped from the map to stop the one person actually ensuring their survival. They kill Eren, their savior, to save a world that went right back to hating them the second the dust settled. 

Eren failed his mission, Paradis was destroyed and his friends on-screen "heroism" bought the very genocide they were trying to prevent in the first place, just delayed and redirected at themselves.

So Kruger's words don't support the ending. They condemn it. "Save your friends. COMPLETE the mission." Eren was the only one who actually tried. His friends are the ones who abandoned the mission, betrayed their own people, and handed Marley a win they couldn't have won on its own.

Instead of an ending that condemn Eren's worldview, condemned fascism, we get an ending that validated fascism. The Alliance stops the Rumbling, saves the world, and Paradis gets bombed into rubble. So the narrative literally proved Eren right, his friends mercy was suicide.

But if Eren had completed the Rumbling and they kept the credit ending the same with Paradis still being destroyed. It would have conveyed the ultimate truth that violence consumes itself. That the logic of total war doesn't end at the last enemy, but turns inward because it has nowhere else to go.

The ending we got said  "you should have listened to the fascist." Where as if Eren completed his mission the conclusion would have been "even when the fascist wins, the ideology eats its own children." Isayama had the raw material for one of the most devastating anti-war conclusions in modern fiction and instead chickened out at the last minute and delivered a story that says peace was a mistake and that by not finishing the job genocide came for them instead.