In thinking about it more, love isn't about the person being loved.  It is about the person doing the loving.  It is not uncommon for a person to love someone that doesn't love them back.  It always has to start out that way because two people can't fall in love with each other at exactly the same moment in time.  One has to love the other first, and then the other reciprocates (sometimes).  If one person doesn't "go first", then love would never happen.  The person going first has to love someone without being loved back. 

So what do you do if the person doesn't love you back?  That depends.  If they are a good person, they won't use you, or abuse the love that you have for them by taking advantage of you and treating you shabbily.  The two of you need to give the other person enough time to either fall in love too, or to figure out it isn't going to happen and decide if the two of you should go separate ways, as painful as that may be for the person who is in love.  I think it is worse for the person who is not in love to pretend that he/she is.  I think it is also important to not try and drive the person who is doing the loving away by treating them badly.  I think it is also important for the person in love to not demand that the other person express feelings that are not present, that is to lie and say that they are in love. 

What I feel constitutes "abuse", is putting your wants ahead of another person's needs (provided you are in a relationship with that person).  I think a relationship can go on for a long time even if one of the parties is not "in love" with the other.  It has to go on for some time because the falling in love isn't always synchronous.  How long can it go on?  I think it depends how long the person not in love can keep working on making the relationship work. 

Love is not the only reason that people have a relationship.  There are plenty of successful relationships based on things other than love.  In arranged marriages, the parties often don't even know each other before the marriage.  In that context, what constitutes a "good" marriage or relationship?  That is really for the parties to decide for themselves.  But if each person is getting their needs met, it can be a good relationship even if they are not in love with each other. 

I suspect that the "hell hath no fury" effect is to protect one from being used and one's love being taken advantage of, that is from being abused.  I have never experience hating a former lover myself.  I don't feel that anyone I have loved has used me, and I have never used anyone.  I have always been completely straight and honest with my partner, as honest with her as I was with myself.  Every long term relationship I knew my partner well enough that she couldn't lie to me, and I was always careful to never put her in a position where she would want to lie to me.  I wasn't going to lie to anyone about how I felt about her, or about anything.  I have spent too much time and money getting in touch with who I really am to pretend I am something I am not. 

I think the "hell hath no fury" effect can be triggered inappropriately.  That is a former lover can be hated with a ferocity which exceeds that which can be inflicted on anyone else, even mass murderers.  I have been on the receiving end of that.  In hindsight, there is nothing I did to "deserve" such hatred.  Hatred, like love is in the heart of the person who feels it.  It isn't about the person being hated. 

Love and hate are mostly projections of ourselves.