It's a good move.
The King can't command when on the front lines, the battle (in its current form) is already decided.
By taunting the King, he has drawn him into an assault where with the super strength gives him the upper hand.
Why risk himself at all? Because the other family may turncoat at any time, and killing the enemy leader would leave them in a far better position than if the battle were to be drawn out and grind to its inevitable conclusion.
Remember, it isn't total war, the king can't just click on his troops and give the order to pull back, and he can't maneuver in a meaningful way anyway. He's about to be encircled and he is weakening the line facing the strongest enemy troops.
Taunting is perfectly valid here.