Relationships do go bad and that is just the way it is. People have very diverse personalities and not everyone is going to get along. Personalities will ultimately clash and people will have different sets of beliefs and morals. This is when it is time to look for someone who is more compatible.
I think if there is hate in a relationship then its a unhealthy relationship. There should be love, respect, tolerance, patience in a healthy relationship. If someone starts to feel a sense of hate towards their significant other then that is a sign to get out right away before it becomes lethal to both partners.
Love and hate are passions and as such somewhat beyond the control of the person experienceing these emotions.
Love makes you feel good in a happy way. It is intense, joyous, and exciting. Being "in love" is an ecstatic state of mind that causes the person who is experiencing that passion to overlook the flaws of the beloved person or object. Hate is just as heated as love.
When a relationship has gone bad, the same or greater intensity of hate is experienced toward the wayward lover. Hate is a passion of anger and as such causes intense feelings of anger, rage, and feelings of betrayal. A person acting out of these feelings is again somewhat "out of control" and may be totally focused on his or her anger rather than on reality.
The polar opposite of either love or hate is simple apathy. Once the intense feelings are out of the way, apathy, or not caring one way or the other, is what is left. Once a person works through the "in love" and the "break up"; the stages of anger and grief give way to apathy.
Love arouses intense emotions in people. When love goes awry, the emotional intensity generally doesn't disappear, and the person suffering must figure out a way to dissipate the energy. If the relationship has gone bad, then often the intense emotion morphs into anger or even hatred. Centuries of poetry in all languages and an entire recording industry has been devoted to decades to analyzing love and relationships. The most notable example that comes to my mind of a singer bemoaning love gone bad is "You Oughta Know" by Alanis Morrissette.
Love and hate are both intense emotions that is quite true, they are two opposite pole. May be a person driven by hate will be as much motivated to harm the object of hate, as the same person motivated by love will be motivated to protect and serve the object of love. But I think here the similarity between love and hate.
People often talk about how the love of a person deceived in love can turn into hate. But that kind of love is not love. What we mistake for love is a desire to own and enjoy something you love.
Love is a noun as well as a verb. Love is also some kind of action. A synonym for love as an action is service. Parents show love for their infant child by serving the child. They do not expect anything in return. For them the joy of serving is a reward in itself. A love of this kind will not turn into hate that easily.
Love and hate are such strong emotions, that they are just as powerful as each other, but one is positive the other is negative. That's what creates a thin line between good and evil. And when it comes to relationships, you have to chalk it up to experience; now you are much wiser and will choose better in the future.