How harmful is it for kids to witness their parents fighting?

My parents fought so much and it was scary, we would cry and he was violent so we worried he was gonna kill her, as I got older I learned it's not right and it needed to stop, my mom has surgery and he started hitting in her, I ran and cut them off and yelled at him to leave, I was 14. I'm 27 and have broken up so many abusive situations, have saved people by opening my home and if anyone dared to come on my property with intent to hard, I would have a gun in one hand and my phone calling the police in another. A man layed his hand on me and I grabbed anything I found hard and went for his knee cap. it can cause alot of fear, some will think it's normal and stay in situations like that, others like myself learned from it, im married to an amazing man that has not and won't ever lay a hand on me, but when we have a little argument i tense up, he sees this and goes, im sorry please don't worry I would never harm you. we have two kids and manage to always go in a different room and speak it out and try to keep it cool, their will always be arguments in a relationship, but should never be a normal thing.