The Unity of the Virtues
I'm pretty sure that Plato was deluded on this point. He seems to have believed that virtue originated in an intellectual appreciation of the form of the best, and that anyone who truly knew what the virtuous course was would follow it. He didn't believe in the possibility of weakness of will or ακρισία, acting against one's better judgement. At least to those of us mortals suffering from that disease, this is a very weird belief to have.
Bad behavior seems to be rampant in men with a lot of power. The most obvious explanation is that as Lord Acton proclaimed, power corrupts. How does that work? I think it works by giving the powerful impunity - too much freedom from the constraints otherwise imposed by society.
A disappointing corollary is that too much freedom can be a bad thing.