So the guards are more hardcore than the Dragonborn? At least they survived to make a career-change.
Something I realized a few days ago. Killmoves apply the enchantment of whatever weapon you have equipped to the poor victim. Ok, so far so good. It's nothing to be surprised about. The silly thing is that the game doesn't account for what the killmove's animation actually is.
Clock them in the skull with your hammer of flaming doom? Crispy fire everywhere.
Headbutt them in the face? You're so hot you light people on fire! Yol, baby!
Toss them on the floor and step on their face? Dragonborn stomps so hard the sheer friction makes them burst out in flame!