I get the sense that some of your understanding of Sausage declaring war on Blue is slightly flawed. You seem to think it's a choice he's making right now, something that could have been avoided if he had decided to do things differently. That there is a beautiful peaceful ending that could be reached otherwise.
But the choice Sausage made that led to this ending happened a LONG time ago. He's been on a fixed path for decades now, and this is how the path ends.
Sausage first learned of the prophecy when he first came to be under Lady Death, when he was 13. He has always known that he will die 4 days after he becomes king, and at that same time, the final battle of the war will occur, and it will end. From his perspective, there are 2 choices. Let Blue Kingdom kill all of them or kill all of Blue. The war only ends when one kingdom is gone, that's the nature of the conflict he was raised to know and fight. So, he obviously wants as many Red Kingdom members to survive as possible.
And he can risk EVERYTHING to make that happen. Helping the Creaking King doesn't matter, because by the time that comes back to bite him, he'll be dead. Doing "bad" things doesn't matter, because it's all just a game of being alive or dead. Nothing after that battle will matter to him, because he'll be dead.
You could argue "Well why would he decide to follow Lady Death?" You could ignore the fact that he had no one else, and that she's the goddess of death, and he was 13. But hey, I like imagining happy endings as much as the next person, so let's explore that hypothetical.
First of all, there's the question of if Sausage would even still be alive. Hell, if the RED KINGDOM would still exist. The last battle ended with just him and Katie. If he could have died whenever, if he was a less powerful mage, would he have made it? Would the Red Kingdom JUST have been Katie? Would she have decided to uphold the banner alone until strangers could come along and help her? Or would she have gone back to Blue? But maybe he'd have lived, or Katie would have stayed.
If Sausage even just a less powerful mage, would Cherri have been sent to train under him? Or would she and Apo have gone elsewhere? Would we have them in the Red Kingdom? But maybe he would have been powerful enough without Lady Death, and they still would have come.
But there is one thing that has NO maybe. If Sausage wasn't a powerful necromancer, if he hadn't been told so by his goddess, then Cam would be dead. The first ruler of Red in our story wouldn't have been alive to be there. There would only be one mage in Red Kingdom.
The story of Bannerfall, as we know it now, doesn't exist without Sausage in his current form. There is no world without this prophecy. It was ALWAYS going to happen like this. It was ALWAYS going to end like this. A physical manifestation of the themes in the story. That the kingdoms must fight, that the war will always continue. It is inevitable.
The only possible end comes from death herself.
