Two specific examples of ways someone becomes a hero

Merlin becomes a hero when he talks the Great Dragon Kilgharrah into breathing onto the excalibur. This is key to defeating Tristan de Bois because it causes the sword to become a weapon that is capable of killing the dead.

Uther Pendragon becomes a hero when he takes the place of his son to die instead; he ends up killing Tristan de Bois.