Just going to mention that no dull sword, or sword for that matter should go through a knights armour, let alone the chestplate of said armour killing the knight. The sword is incapable of doing such a thing and would break trying. He could have killed him logically by going for an open spot, like the whole head area.
Swords don't cut through what swords are made of. Axes have a hard enough time of it. You would need a solid, thick, spike of metal and a LOT of strength to puncture metal without it bending or breaking. If he had grabbed the magic dagger i could see it working of course. Its a super weapon after all. You can definitely get through bronze with steel using enough force and a strong weapon.