Hi OC,
It only goes to show the expanse of people who are effected by one person's actions. It sounds like you believe your friend is in jail but she does not deserve it because she was mentally ill at the time of her actions? I'd feel upset, I think, in your shoes. So sorry ....the law sucks sometimes, I really think.
The explanation I am drawn most to is that sanity/insanity is a sort of sliding scale where most people are not actually or always sane or insane. In other words one is not totally sane all of the time nor completely insane either (in most cases??).
That means we all have insane moments and some longer than others.

It's kind of scary to think about that but it makes sense to me because there have been days!!!!

The term mentally ill, imo, is simply a description of a person who has slid further along the scale toward insanity and remains there, longer than usual. Some mental illnesses, though, are not curable so that must mean the person stays "more" insane than the rest of us, until such cure is found, or until the problem is erradicated (the problem being the cause of the mental illness and then there is the matter of treatments of it's effects).
The term "snapped" I have a problem with sometimes because I really believe, in the split second before action, when the compulsion to act is sooooo strong......a sane person makes a choice. So where does the snapping come in? It seems to imply a lack of control over actions and I just don't believe that to be true, a lot of the time. People choose how to act. They may not think about it very long, they might not consider the consequences, or the morality, or other factors about their actions, but they do indeed decide to take action. I think most sane people do anyway.
Legally, a person can be found to not be accountable for their actions due to insanity (which is supposed to include the idea that the person has no judgement....cannot tell the difference between right and wrong.....is not able to process their own behaviour on a moral/sane scale (at least at the time of the offense).
So this may be what you mean by "snapped"....in that for that time period, your friend did not process her actions sanely......was not able to process them due to mental illness?
This is the tricky part.......
Could she have known about post-partum depression and expected to use it as a defense? (which didn't work out in her favour, as she is in jail now).
Or did she indeed move so far down on the insanity scale that she was truly not able to think rationally or judge her own behaviour?
And how can this be determined?
I think, in law, the court tries to consider the person's actions/behaviour before and after the crime....to see if they were acting sanely at those times and how much of the time as close and just after the crime as can be examined.
Does a person "snap" for a second? Or......for a second, does the person choose to ignor all he/she knows is right and acts because he/she feels compelled to/wants to very badly? Or is there a mental illness present that prevents the person from choosing?
A lot of questions I sure don't know the answers to but my own experiences of feeling at the brink of insanity have taught me there is a very fine line between the two and so far, I have not chosen to cross the line, although it has been tempting at times. I must not have been mentally ill enough to have forgotten the difference between right and wrong and I know I didn't lose my ability to choose.
In cases such as Dahmer's.......I fail to see how he could be sane. Yet, in Bundy's case, his actions by living a double life, having a family and all that....seem to indicate sanity, one might think.....?
On the lighter side, here's a test that prooves I am totally insane: (it only works if you have the sound on tho

):
http://www.tekzoned.com/instest/
Sela