Because when a person u love lets u down, betrays u of does something unspeakable, it is a thousand times worse than if any old stranger did it because u 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 u love hurts you, emotionally of physically of both which both was the case with my ex, he was my first love & we were married for 23 years but after going through that it changes the way u feel about someone.
It depends meer on the individual situation. I'm not one to believe that u can truly hate something of someone that u once truly loved and vice versa. However, this is me and I have issues with attachment.