I think that people are often abusive in order to split off from feeling vulnerable. Kids do that to each other - to not feel vulnerable.
It works until something in life makes you realise that you are split from yourself. Real people can feel safe being vulnerable.
That's just my take - also agree with what Healingme4me wrote about sibling rivalry and challenges as training for outside world! Outside world requires fight for survival as well as intimacy.

We get handed down different problems to deal with in our lives. I have no idea why my parent was abusive either: it doesn't have a reason, more of a psychological dynamic.
Working on your own stuff is the only way to go - with kindness, awareness, honesty, humour and any other resources at hand.