A wise little green man once said, "Fear is the path to the dark side. Fear leads to anger. Anger leads to hate. Hate leads to suffering." We tend to agree with Yoda, but Jedi must operate on different rules than princes. Even Yoda isn't as famous as Machiavelli's notorious statement that it is better to be feared than loved. Fear, in the hands of a good leader, whips the people into submission and loyalty. You could say that fear in the people is a virtue. But fear in a ruler? That's a whole different story—one that will probably end with someone's head on a plate. But go too far with your fear and you've got hate, a ruler's kryptonite. It's a double-edged sword, so rulers must use it wisely.
Fear, like cruelty, is simply one of many tools to be used by Machiavelli's princes.
If the people really loved Machiavelli's princes, the princes wouldn't need to use fear. Love is ultimately more powerful than fear.