So, What Is It?
To me, the heart of Machiavellianism is defined by these concepts:
Relativism - Machiavelli believed that every person and every situation was unique. While some broad principles can be applied to many situations, one must always consider the unique personalities involved and the unique circumstances of time and place. Nothing is true for all people in every place all of the time. Everything is relative to everything else.
Individualism - Machiavelli wrote The Prince to one man - the Prince of Florence. He advised that man how best to govern his principality. He would have, I believe, given different advice to another ruler in another time and place. He would surely have given different advice to the King of France or to the Pope than he gave to the Prince of Florence. Goals, to Machiavelli, are self-determined, not universal. He would have given different advice to a blacksmith or a priest. There are no universal rules that apply to all humans, except for a few having to do with continuous breathing, eating, gravity, etc.
Subjectivism - I think the purpose of life is this and you think the purpose of life is that. I think love is this and you think love is that. I think humanity originated in this way and you think it originated that way. We all live in our own reality. I think my writing is brilliant, while others (all fools!), may think my writing is crap. We can agree on some things, but not on all things. The point is not to compel all others to agree with you about everything - which is impossible - but to pursue your own goals as you define them. For example, Machiavelli argued that the common people want freedom, security, and autonomy while the wealthy and powerful want to oppress the common people and deprive them of freedom, security, and autonomy. What you believe is based on who you are: your genetics, your family, your country, your culture, your social class, your level of education, books you have read, and your experiences.
Rational Thought - Machiavelli was a keen student of human nature. When he described a thing, he told us what he had seen, heard, and concluded. He did not tell us what the Church taught about human nature, what the Bible said, or what Aristotle said. He did not quote St. Thomas Aquinas or St. Augustine. For example, he pointed out - without mentioning the Ten Commandments - that lying is universally used by human beings, especially powerful and successful human beings, to achieve their goals. When one chooses to lie, he observed, it is best to lie skillfully. Is this not rational?
The Ends Usually Justify the Means - Machiavelli was a Christian, but he argued that the image of the Christian warrior or Crusader was better for Florence than the image of a passive Christian saint praying for salvation. He did not praise Christian saints who knelt and prayed in the Roman Colosseum as the hungry lions ran toward him. He had observed that swords won more battles than prayers, and he wanted Italy's churches to build martial and manly values in Italian men, not make them into wimpy martyrs who would die for God - but not fight for Italy. First, think of your goal (like a city of strong, trained men who would fight when called upon). Then, figure out a plan to accomplish your goal (the Church should emphasize the image of the strong, male serving God by fighting for his city).