The authors are quite convincing in their portrayal of Hitler and, especially, Napoleon as Manic Depressives. Their case of Stalin, however, is completely unconvincing; Stalin clearly was a psychopath.
Napoleon was a charismatic leader, but he completely abandoned his army both in Egypt and in Russia. With the bi-polar disorder, the authors have found a convincing explanation for the extremes of his behavior.
In the case of Hitler, I want to refer to the book "When Illness strikes the Leader" by Post and Robins. They describe how Hitler was taking coke and speed simultaneously. Cocaine produces a sense of well-being and confidence that can mount to feelings of elation and grandiosity. Hitler's doctor, Morell, administered his notorious patient a wide array of sedatives. Hitler confessed: "If I had not got my faithful Morell, I should be absolutely knocked out!" Hitler's addictions could also explain his successive symptoms of mania and depression.