Sqad - first of all my sincere symnpathy for your situation - I too suffer from clinical depression and have been hospitalised, and am on medication, so I do know exactly how you feel.
I can see that you feel that your illness does not mitigate cruelty because you feel that it is not an action you would commit.
I understand that too - even though i was hospitalised for three months, my illness only directed my cruelty inwards, and on the occasion it manifested itself outwardly, it was in the form of verbal cruelty towards my nearest and dearest - a pain i will carry for the rest of my life.
However, our condition affects us in different ways, and as you rightly point out, no-one can second-guess the circumstances in this case - or any other.
I do take your point that you would not behave this way as a result of your feelings, and i entirely concur - but others are affected differently, and while illness does not ever excuse cruetly, it can explain it, and we as a society should use or compassion first and our anger second, in working out the best ways of dealing with instances like this.
Was this woman's behaviour excused by her illness - no. Was it perhaps explained? Ithink so.