Musar for Sotah 82:8

The ninth category of flattery is he who honors the wicked because he wants to preserve peace. It is true that he does not speak good of the wicked and he does not conduct himself in any way that would cause people to think that the wicked person is honored in his eyes, for he does not show him any honor except in the way that people honor the rich, because their path has been prosperous, and not because of their own merit. Yet even in this there is sin and guilt, for it has never been permitted to honor the wicked, except for terror, that is to say, if one is afraid that the wicked man will injure him or cause him a loss at a time when the hand of the wicked is mighty. Therefore it was permitted to honor the wicked man as people respect all men who are mighty, but he should not praise him and should not speak good of him to people. And so said our Sages, of blessed memory, "It is permissable to flatter the wicked in this world" (Sotah 41b). But there are wicked whom we may not flatter. Whence do we derive this? From Mordecai, who was advised to flatter Haman, to which he retorted, "Thou shalt not seek their peace nor their prosperity" (Deut. 23:7). And they would say to him "Our Rabbis taught that we must flatter them for the sake of peace," but even then, Mordecai did not want to flatter such a wicked man as this.