How does Machiavelli justify why a prince should not always act based on ideals of virtue?

I found a very interesting article in The New York Times Learning Network. Check out the excerpt below.

Machiavelli teaches that in a world where so many are not good, you must learn to be able to not be good. The virtues taught in our secular and religious schools are incompatible with the virtues one must practice to safeguard those same institutions. The power of the lion and the cleverness of the fox: These are the qualities a leader must harness to preserve the republic.

Machiavelli has long been called a teacher of evil. But the author of “The Prince” never urged evil for evil’s sake. The proper aim of a leader is to maintain his state (and, not incidentally, his job). Politics is an arena where following virtue often leads to the ruin of a state, whereas pursuing what appears to be vice results in security and well-being. In short, there are never easy choices, and prudence consists of knowing how to recognize the qualities of the hard decisions you face and choosing the less bad as what is the most good.