I'm sure he cared about the fallen naaru but knows it has to fall because it is a being of the void and it would cause terrible damage. The world soul of Argus had to be stopped because it was a regenerator for the Legion. I don't know where you think he didn't care. Yeah he sighed at the end of Legion because the Burning Crusade is more or less over.
That same crusade that hunted down his people across the stars.