For Michel de Montaigne, having an essay about cowardice and another about cruelty was apparently not enough, he also needed one where he declared cowardice to be the mother of all cruelty. His argument was that sentimental affectation, the kind of personality that would make someone weep at the most maudlin music or story, also leads people to perform especially cruel acts, to make up for the weakness they can’t help but display at times.
This position allows Montaigne to also make the opposite claim, that people capable of handling their emotions and not being manipulated are also capable of granting clemency and kindness towards defeated foes. He is arguing that cruelty, contrary to widespread belief, is not a sign of strength, but of the greatest possible weakness.
In short, Montaigne is saying that emotionally mature people are both the strongest and the kindest people and that leaders who lash out at opponents and seek revenge at every turn are cowards worthy of our deepest scorn.