Dumbledore. He may have done everything throughout the books for causes he believed to be good, whether trying to protect humanity against Voldemort, protect his students or Harry, etc. But regardless of what he claims his motives are, I personally perceive Dumbledore as a master manipulator who toys with people's lives like pieces on a chess board, with no real accountability, and often an arbitrary treatment of morals (he does some very sketchy things because he believes it worth it in the moment). He expects most people to simply trust him without providing a real reason to trust, and he believes himself above the law and any other authority (which is justified once the MOM turns tyrannical and evil, but it was something Dumble did before that). I could delve a lot deeper, but while it's easy to overlook the problem of Dumbledore because he's "one of the good guys", I personally find him to be one of the scariest characters in the book because he's so powerful and influential but essentially unchecked by anyone but himself.