Incongruity is the essence of humor; contradiction is the essence of evil. The formal cause of evil is the contingent being rejecting its contingency. It is humorous to joke about wanting to kill your mother-in-law; it is evil to actually do so. Rudeness and insults are quite often funny when intended as a joke/teasing, but become mean when taken seriously. The incongruity of sex in certain situations forms the material for dirty jokes; yet to take such seriously is called fornication. (Likewise, to take sexual pleasure from such jokes of a sexual nature – and not just that pleasure of incongruity – is fornication of the heart.)
To take an example, Sunday in G. K. Chesterton’s The Man Who Was Thursday is an “anarchist” leader, yet in an entirely playful manner. It is Gregory who takes it seriously, and who is the evil one.