You propose a friendly agreement: you say to the vicious person you want to mock, "you are such a kind soul."
You know the kind soul is smart enough to realize you don't mean what you say.
He wants to believe what you say, at the same time he knows he is an accomplished hypocrite, and you have caught him out.
But he has gotten used to pretending to be what he is not and it has become second nature.
You suggest to the kind soul that he relies on the power he feels in being taken for what he is not, and he is vulnerable because of it. He wants both you and he to be deceived, but you are not. You suggest he is in the habit of feeling safe when people are making an appearance of being agreeable, but he is not safe with you. The ironist uses the power of his victim against himself.