I had something in mind, a scenario where back during the war between humans and monsters, Asgore (as a child or teen) is accompanying his father, the previous king, as he leads his forces into battle, and he witnesses his father get struck down by an enemy. While standing there, trying to get his senses back to normal, he suddenly goes berserk, fire magic pulsing all across his body as he whirls on the human soldier, unleashing his destructive power upon the unfortunate sonofabitch, scaring him almost literally to death. The human forces, seeing this, decide to retreat, and then the monsters proclaim Asgore as their new king.