jessica silva stabbed james polkinghorne outside her home in 2012 .
silva was physically and mentally abused by her husband polkinghorne .
she claims that if she had not acted first she would have been killed .
silva says all she wants from her dead husband 's family is ` forgiveness '
polkinghorne 's father can not understand why she killed his abusive son .
jessica was found guilty of manslaughter but had sentence suspended .
justice hoeben , who presided over the case , called situation ` exceptional '
jessica silva story features on sunday night 's 60 minutes on nine network .