Okay, so you may be thinking that it's an easy decision to make, right?  If victims are treated so horribly, why wouldn't they just leave?  It's not so easy...

There are many reasons why victims of abuse may stay in abusive relationships.  The idea that they don't want to leave or that they enjoy the abuse is a misconception.  They live in terror and fear, which is what their actions of leaving are controlled by.  Also, the victim genuinely loves their abuser in many cases.

What?  Really?  Yes.  Abusive relationships do not start off abusive.  They begin as any other normal relationship...the romance, the compliments, the whole shebang.  As time goes on, though, the controlling and abusive behaviors take over the romance.  Victims are in love with the person their abuser was at the beginning.  They may not want to leave the relationship, they want the abuse to stop.

Some reasons victims stay may be:
Economic dependence
Fear of emotional damage to children
Lack of job skills or education
Social isolation resulting in lack of support
Fear of retaliation from abuser
Religious convictions
Fear of loneliness and being alone (as the abuser has ensured they feel worthless and unwanted)
Fear that abuser may not be able to survive without them
Guilt about failure of marriage
Belief/Hope abuser will change
Victim and abuser have children together
Victim loves abuser
Fear of greater danger to self and children

In fact, the risk of injury or death increases 75% when a victim leaves an abusive relationship.  Also, a victim--on average--will leave an abusive relationship a minimum of 7 times before finally breaking free.

This is a situation that, unless you have studied the inner-workings of it or experienced it personally or vicariously, there is no way to completely understand.  What we do know, however, is that if a victim has not only family and friends who support them but also a community who supports them, they have better chances of breaking free.