Machiavelli was not being sarcastic or a moralist; he was being honest. From his observations, he wrote what he percieved to be true -- this is how leaders operate, particularly the Borgias of his day. The timelessness of his work suggests his observations were correct, much as though we may not agree with them (but those seeking princely power undoubtedly do.)
The basic point of Machiavelli is that the ends completely justify the means. And if the end is to keep and extend power, then whatever it takes to do this should be employed. Morality does not factor. The goal is to rule and all things are used to reach that goal.
With this said, there is another interpretation to the work. Some state that Machiavelli is being sarcastic. In other words, it is a work of showing the horrors of such a rule to dissuade people from seeking power. There is some merit to this line of thinking.
It's true that the "Machiavel" became a common trope in English plays, largely due to the Prince, but Machiavelli wrote many other works than The Prince. His Discourses on Livy, in particular, deal with the tendencies of republican governments (which he favored) to decline over time due to corruption, a theme that remained prominent in republican political discourse for centuries. Many historians of political thought have argued that the Prince offers a way to provide stability in the face of corruption by defining virtue as basically raison d'etat. The republican strand of thought that Machiavelli embodies was influential on a generation of Americans, particularly James Madison. That doesn't mean it's not repressive, but that Machiavelli's thought is more complex, perhaps, than is often popularly portrayed.
The earlier answers are all very helpful. Machiavelli was particularly despised in Renaissance England, at least by the writers of many plays. The stock character of the "Machiavel" was a character who deliberatly manipulated others for selfish reasons. Such characters were often seen as almost Satanic.
I have to agree with the previous posts. Machiavelli's text was considered a "handbook" on the "how to" keep one's position of authority. Therefore, some who used his text would only be concerned with keeping their power (regardless of what it took). Some of the "suggestions" made were seen as oppressive.
Whenever I think of Machiavelli I think of a couple of his famous "adages" that, if are not his word for word, have come to be known to come from him. The first is "the ends justifies the means (as long as the ends are justified)." The second is "it is better to be feared than loved." Those ideas, especially considered together are the basis for this new kind of leadership he expounded.
Before Machiavelli, works about governing tended to emphasize morality. This changed with Machiavelli. He was interested only in how a ruler might keep power. This is Machiavellianism -- it can be seen as the ideology that holds that the only goal of a ruler should be to keep power. It is acceptable to do this by any means necessary. This is why it seems like such a ruthless idea.
Machiavelli's purpose was to provide a guide, if you will, to how a ruler could go about assessing any situation and determining the course of action that would be the most advantageous for himself. Considerations for his subjects or foreigners were irrelevant; the goal always and only was advancement of his own interests. The complete lack of interest in providing any assistance or creating any benefit for those he ruled certainly made his philosophy repressive; the ease with which approaches or actions could be changed if needed in order to benefit achievement of his end goals made it very manipulative.
It is interpreted as a repressive manipulative strategy precisely because Machiavelli wrote this book as a kind of guide to princes on how they can maintain and increase their power through manipulation of their populace. Therefore it contains various methods that show the cynicism of Machiavelli and how to manipulate the common man by, for example, treating them well at one stage then treating them harshly depending on the context