What is Darth Vader's diagnosis?
What some folks miss is Anakin Skywalker never had a pure Jedi education. From the start, Chancellor Palpatine was teaching him, influencing him. People ask how he could all of a sudden turn Sith, but at the risk of being contradicted later by Lucas, I believe he was being taught the principles of the Sith, as he was being educated in the Code of the Jedi.
There are lines in either Episode Two, or Episode Three, where Palpatine goes something like "Remember what I taught you, those with power are loath to part with it.". That sounds to me like he was giving Anakin lessons about power and its uses, a rather Sith thing to be doing. Why can't the secret Sith give his pupil in politics shadow lessons in Sith doctrine ? He wouldn't have to announce it, saying, turn your Sith Textbook to page 108, he could simply give those lessons as pointers for dealing with political situations, or dealing with the real world, as a counterpoint to what the Jedi taught him. Rather than only having a few hours to teach him everything, Palpatine would have over ten years to instruct his pupil in Sith thought, if not actual force techniques.
There was always a parallel path. What Palpatine did over time was force Anakin into the belief that he had no other path to walk than to walk the path of Dark Side, and as he was tunnelled more into that, his behavior became increasingly outside the pale.
Anakin doesn't have to be mentally ill to react like he did. People underestimate the consequences of having a head full of bad ideas, of being under the bad influence of somebody who could be termed a psychopath, a remorseless manipulator.
