I know you're looking for "psychohistory" but I am afraid that I'm going to have to answer this question in a different sort of way. Long story short, the idea of psychohistory itself is not only bad history, it's bad psychology. Although as a concept we can certainly talk about what makes people make certain decisions, us v them sort of thinking, and mental illness when it actually does exist (although even these instances are often debatable). However, a lot of times, it tends to take the stance that bad things happen only happen because people or evil, or are suffering from some mental trauma or illness. It assumes that only bad people do bad things and only good people do good things--the just world hypothesis.. Furthermore, such a belief tends to hold that when people do evil things, Hitler for instance, it's because they are suffering from some sort of psychological problem, which is oft times not correct.

Another problem of psychohistory is that we can't actually know the motivations of the historical people who we're analyzing. We can, if and when they write their motivations and feelings down, but unless we're able to physically sit down with them we're just left with whatever they happened to write during their lifetime--which is not sufficient for any sort of diagnosis in the realm of psychology. We can certainly make educated guesses, but are still unable to fully assess the he mindset of person we wish to analyze because they are no longer alive.

As some of the other comments here have alluded to, psychohistory is somewhat problematic and it's very passe in the academy. One of the few areas in which psychohistory is present (or elements of it) is within early modern historiography. An example of a relatively recent (1987) psychobiography is William Bouwsma's John Calvin: A Sixteenth-Century Portrait. Even when we took it apart in our seminar, the general consensus was that Bouwsma's biography is an intellectually stimulating endeavor.