If the strength of both people were at their peak, then even if their abilities were comparable, a full-on clash wouldn't lead to mutual destruction, at most both would be severely injured, and perhaps even a slight disparity in their strength might determine the outcome.
But Mu Hanyan's meridians were shattered and qi sea ruptured, and that mysterious power came so abruptly that she had no time to think, and directly slashed with the sword, unable to wield the power freely. Moreover, once she unleashed the Destruction Sword Dao, her mind would be overwhelmed by intense desires for slaughter and destruction, making it impossible to maintain her usual calm.
