Because when a person tu amor lets tu down, betrays tu o does something unspeakable, it is a thousand times worse than if any old stranger did it because tu trusted them. And when that trust is chucked back in your face, the worst kind of hatred is born. Emotional damage is some serious stuff :|
I don't per say despise them but I know I don't like who my ex became, it can happen when someone tu amor hurts you, emotionally o physically o both which both was the case with my ex, he was my first amor & we were married for 23 years but after going through that it changes the way tu feel about someone.
It depends más on the individual situation. I'm not one to believe that tu can truly hate something o someone that tu once truly loved and vice versa. However, this is me and I have issues with attachment.