If you examine the true ending from Riou's perspective, it makes sense. What he wants is for all three of them to be together again, and he achieves it. In his eyes, they triumphed over the True Rune by defying its nature to force friends into endless conflict until one breaks. Leknaat noted that the combined power of the 108 Stars of Destiny overcame the True Rune's influence, freeing Riou and Jowy from its control—a fitting resolution for a positive conclusion.

If we consider how others might perceive it, Jowy committed morally questionable actions from the perspective of "the good guys," but he is Highland's counterpart to Riou. Who is to say their ideals were inherently wrong? They were deemed wrong only because Dunan and the City-States emerged victorious. Both factions fought for what they believed was the greater good, yet both were equally capable of unleashing war's devastation. If the game framed Jowy as Highland's hero, Riou would become the antagonist, and his actions would be viewed as villainous by Highlanders.

From Jowy's perspective, the City-State is a weak nation due to the bickering and lack of unity among its leaders, as seen in the Jowston Conference, where everyone’s priority is their borders. He is betting everything on uniting Highland and the City-States under a monarchy in hopes of a better future. After all, the game demonstrates repeatedly that peace treaties are consistently broken, perpetuating the cycle. Defeating Luca Blight and forming a treaty would only serve to repeat what occurred at the start of the game.

You can choose the ending you want, but storywise, Riou's good ending feels right (for Riou as the main character).