First of all, welcome to BPO. I am sorry that it was such sad circumstances that brought you here.
Sudden aggression is usually caused by a medical condition like others have mentioned. Before putting her down, you should give her a full work-up. It might be that she has something as simple as an infection that you can treat and then the aggression will go away when the pain is gone. It might also be something that's not so simple (i.e. a brain tumor). You really don't know what can be done about the aggression unless you know the cause. My saint bernard has neurological issues (the neurologist thinks its a brain tumor) and she has sudden un-provoked boughts of aggession at night now but still has never bit anyone. There really isn't anything we can do except give her the best quality of life possible.
