What happened to Mugaro was tragic. However, it has been hinted by King Charioce XVII’s butler in episode 19 that King Charioce XVII needed to attend the ball regardless of the feelings due to needing funds for the coming war. His butler says, “I understand your feelings, but with the uncertainty of the coming war, fund-raising events are part of the king’s duties.” The coming war was going to happen, but when it was going to happen, we did not know. However, if the coming war was going to happen, it would have happened differently. In other words, had Mugaro not been killed and went back to the Gods, it is likely that Gabriel will try to manipulate Mugaro into going through another war and force Jeanne D’Arc to join forces with the gods, angels, and Mugaro in “saving the world”. Honestly, Mugaro would’ve likely been injured in the coming war (or likely died depending on how bad the humans’ weapons will hurt him). If the coming war does happen and Mugaro does participate in the war in Anatae, he could have died quickly if Charioce XVII brought the dragon to Anatae. Mugaro’s power does not hold against Bahamut, so he would’ve died in an instant. That’s why it makes sense that the writers would write the scene differently with Mugaro dying from being stabbed by Alessand. Mugaro’s death was tragic, but it is understandable why his death was going to happen regardless.