Monarch High Heaven couldn't believe that someone out there was falsely accusing him of assisting the Shadow Lurker in terrorizing the Supreme Beasts. His anger and confusion showed on his face.
He had to ask, "Why would I do such things? What could I possibly gain from it?"
He seems to be genuinely perplexed and shocked by the thought that he would try to harm the Supreme Beasts who are trying to interfere with his interests during the era of conquest. Apparently, that kind of action feels so alien to him that he couldn't possibly have entertained the idea.
If there is a reason for him to frustrate the expansion efforts of the Supreme Alliance or reduce their influence in the high heaven realm, it is not clear to him.
And no matter how clear those reasons may be to others, he will never admit to seeing them. Much less admit that they are his motives or that he indeed has been frustrating their expansion efforts.
