I'm not a psychiatrist, but I work for one. I think y'all are confusing "evil" with "psychosis". Post-partum psychosis is worse than your basic post-partum depression, but both are very real. Clearly this woman was in a psychotic state. (A quick listen to the 911 tape makes that obvious.)
Does that mean we should give her a big hug and send her on her merry way? Of course not! I agree with the concept of "guilty by reason of insanity" (credit to Bill Maher for that one). Whether found innocent or guilty, she needs to be subjected to heavy psychiatric treatment, completely separated from the rest of humanity for a very long time, if not forever.
What is "evil" anyway? A guy who stalks and harms a woman he barely knows has to have some screwed-up brain chemistry to be in that state. That's not an "excuse", but it's a "reason".