I've known a few in my time. They're unfortunately rare, in no small part because the bad ones don't like the competition.

[And because being a good leader in one arena doesn't always translate to having the skills to thrive in a different one. Ned Stark had been a good leader, but completely unequipped to survive King's Landing. His son seems set to follow in his footsteps. And he's entirely lost count of the examples dashed upon the rocks of history.]

Power makes it easier to be evil. And being evil makes it easier to gain power. At least in the short term.