女が「どちらか一方が死なねば恥をすすげない」と言ったことで、プライドを刺激されたから。

I killed him. But do not look down on me as a mere wicked bandit. I was captivated by that woman's beauty, driven by an impulse that I could not help but satisfy. I tricked the man, bound him, and took the woman before his very eyes. The woman broke down crying, but eventually showed her fierce temperament, saying, 'Unless one of us dies, my shame cannot be washed away.' Stirred by her words, which pricked my pride, I untied the man and crossed swords with him in a fair fight. After a fierce struggle of over twenty clashes, my blade pierced his chest. The woman fled in the confusion, but I have not a single regret.