From this inspiring article:

In the words of Machiavelli in The Prince (chapter 17), "A prince must not mind incurring the charge of cruelty for the purpose of keeping his subjects united and faithful; for, with a very few examples, he will be more merciful than those who, from excess of tenderness, allow disorders to arise, from whence spring bloodshed and rapine." Whether we are dealing with subjects or enemies, and certainly enemies bent on our destruction, an excess of tenderness only leads to a loss of mercy. In the words of Midrash Rabbah on Kohelet 7, "He who is merciful to the cruel is destined to be cruel to the merciful."