Oh I fully agree, but this only happens when you're aching so much all over that you no longer have control over it. The moment you're in so much emotional agony, that you just accidentily end up hurting someone in the process. This is why I'll try to isolate myself when that happens. And then you have those that are too scarred and scared to go back to the previous state. I personally cannot do that. I can't stay in that paranoid state. But I think depending on the person and the kind of abuse they suffered...being jaded and paranoid can be a shield. And you'll lash out at people, not to hurt them or hit them, but to keep them away, so they cannot hurt you.
You ask 'how can anyone do that?'. I ask 'how can you not see that this is a survival mechanism that is very effective?' I'm not talking about happiness, quality of life, or what not. But just the right to not suffer, or at least, not suffer at the hand of others.