Since when does an urge to do something destructive give one some legal right to do it? We all have urges, our legal and moral obligation is to suppress the ones that are bad and do the right thing. There is no illness or disease that makes people kill, especially their own children. Jeffry Dahmer, Charles Manson and John Salvi were notorious schizophrenics, but it didn't help their cases any.
I never said it gave any legal right. As you said, we all have our urges. I had a business partner at one point who triggered an urge to slam an ax blade thru her forehead, when I discovered she was embezzling from me and planning to take control of our venture away from me. I controlled my urges, and may friend with post-partum depression controlled hers.