The dictionary defines Machiavellian as follows:
"being or acting in accordance with the principles of government analyzed in Machiavelli's The Prince,in which political expediency is placed above morality and the use of craft and deceit to maintain the authority and carry out the policies of a ruler is described."
In other words, means justify the ends. Becoming politically powerful is the end goal and far more important than attaining that goal in a moral and honorable way. If you need to be charismatic, be charismatic. If you need to be manipulative, crafty, and deceitful, well so be it. The catch though is that the people a ruler is trying to gain power over can never know about those less than honest methods. The ruler should appear good in public, but in private, anything goes.
As King Edward IV's youngest brother, Richard is not likely to attain the throne through a normal course of events. Richard first gets his older brother George thrown in jail. Next he some how woos Lady Anne into marrying him because it's politically advantageous. Then Richard hires thugs to murder Clarence. At this point, the trail of bodies begins to build up. Richard orders the deaths of just about anybody who could stand in his way to throne (or keeping the throne after he gets it). He has young kids killed even. Now his good outer appearance does begin to fade, but in Richard's own words, he does believe he is able to cover it at times.
But then I sigh; and, with a piece of scripture,
Tell them that God bids us do good for evil:
And thus I clothe my naked villany
With old odd ends stolen out of holy writ;
And seem a saint, when most I play the devil.
I can add colors to the chameleon
Change shapes with Proteus for advantages
And set the murderous Machevil to school