They do this because THEY themselves were somehow mistreated or abused...even though to outsiders, it may not have seemed that way. I have seen people struggle to overcome the affects of this, and some are successful. No, it doesn't work like: well, they know what it felt like, why would they do it then??!! ... its a reaction, I believe. And some people are simply born in a way that seems to make 'determination to be improved and better' an easier thing.
Several weeks ago, I yelled at a lady who was really abusing her small terrier dog...I threatened to call both animal control and the cops... she was totally taken aback... several times after that I saw her in the coffee shop and something about her 'reached me' and as if I could see her pain and confusion. I regret not talking to her nicely...so I learned a lesson from that.
You don't 'war' to make peace...