Child abuse and neglect ?

Yes it can long lasting mental issues.  And why.. unsure to be honest.. but maybe because they themselves were abused.  Does not make it right.. but its what I think.

People abuse because they have inner pain that they do not know how to cope with.

So they repress it.

And the repression doesn't make it go away .. it just builds up tension.

And the person "snaps".

The more abusive someone is the more inner pain they have and the more helpless they feel inside.

So those who are abused as children, usually turn out to be abusive when they grow up too .. because they acquired a lot of emotional pain while growing up and didn't have good parents to help them learn how to cope with their emotions. 

Therapy can help, but some people never find the courage TO heal themselves.