Concerning Lady Macbeth and psychiatry, first of all, Lady Macbeth is not a fiend, we don't know anything about her childhood, and she is not an actual person. She is a character in a play. Anything other than what we see in the drama is mere conjecture, and is silly, since she's a character, not a person.
Second, we don't really even know much about her sanity or insanity, since she was asleep and sleepwalking when she rubs her hands, etc. Sleepwalking doesn't make you insane. Certainly, she is suffering from guilt, but that doesn't mean she's insane, with all the connotations of that word. She commits suicide, of course, but that makes her a victim of mental illness, not a victim of insanity and, again, of all that word suggests.
Also, Lady Macbeth isn't that ruthless, since she can't even kill Duncan herself when she gets the chance. Plus, she pleads with the spirits to unsex her, showing that she even doubts herself. One doesn't pray for ruthlessness, if one is sure of how ruthless you are.
Lady Macbeth should not be described with hyperbole and should not be demonized. She is wicked and ambitious, etc., but she is not absolute evil incarnate, or whatever.
Finally, one aspect of what we do see of her that certainly relates to modern psychiatry is her exhibition of O.C.D.-like symptoms while she is asleep and walking. It seems to me that Shakespeare may have been ahead of his time when he added this detail.
To put it bluntly, Lady Macbeth is the epitome of mental instability. That is about the closest you can relate literature to psychiatry. As her character develops, you can see a person spiraling out of control and imploding to her own macabre and insane mind.
To top it off, she is a manipulator, a plotter, a blood thirsty ambitious fiend that stops at nothing to get her way, a woman who insults and bullies her husband, who hates everyone and, in the end, goes crazy because of her own demonic ways.
Like the previous poster said, you can make all kinds of conjectures on where this behavior came from by making a concise analysis on Lady Mac in every part of the play.
Psychiatry is highly relevant to literature, and this is a fascinating area - Lady Macbeth in the play by William Shakespeare is a very good example. Psychiatry research very strongly suggests that out of all the experiences we have in our lives, our childhood times are the most formative and influential. Out of this time, the first few months of infancy are crucial. Literature gives a unique insight into personality.We don't get to find out much about Macbeth or Lady Macbeth's childhoods, but we can make some intelligent guesses based on what he tells us in the text. One of Lady Macbeth's strongest cries is 'unsex me' when she is expressing her wish to be more ruthless, more male. It may well be that she was attracted to Macbeth originally because she recognised in him something she could mould, manipulate and control. One childhood possibilty from this is a childhood which involved a parent who was either over-controlling or very passive.