At the very least, if it can be proven this woman knew your ex-wife had that allergy, she should be removed from society, permanently. If there is any justice, she'll be verbally shamed on a daily basis, for her despicable behavior, or otherwise made to regret her actions.
To be fair, there are reasons why people are the way they are. This woman may have been abused, for instance. Maybe there are genetic flaws, etc. In a way she is to be pitied for her miserable existence. There is no way for us to sort all that out. Some things are beyond our understanding.
We DO know your ex-wife and her husband were trying to help a friend in need, and this is the thanks they get? There are no words.
IS there such a thing as "cosmic justice? ARE good deeds always rewarded, and bad ones always punished, no matter how it appears? IS there, in the end, always accountability for the things we do? I have to believe there is. You're free to believe what you will. Meanwhile, back here on earth, in this particular case, we have the RIGHT to make her suffer, in equal measure, for what she's done, to the extent we can figure out what that is. That's all we can do.

It certainly sounds like attempted murder at least, even if it may be difficult to prove actual murder, she should certainly face the full brunt of the law.

Oh, no...how horribly sad!!! I'm so sorry for you! I wish I could help in some way. Has the nutcase tenant been arrested yet? It sounds like premeditated murder. How old are the children?