As another perspective to offer, I was the child in this dynamic growing up. My father was verbally / emotionally abusive to us / around us / to my mother. We were constantly on egg shells as things could go from laughing and joking one minute to screaming in our faces the next. He would then want it all to be forgotten or would go in a dark mood because one of has gone and ruined things. It affected me hugely in my childhood, living this way, and it's kind of come back to affect me now I am an age where I could have children the same age I sort of realise afresh how absolutely awful it is to treat children that way. The overhwelming feeling I have when I remember it all is humiliation I think.
In my case there was also a fine line between whether it would also be classed as physical abuse (I think when I think of my own situation I'm unsure whether that's over exaggerating but when I think of it happening to another child I know i would think it was) things like a 'smack' on the head delivered in temper, or once I spilt a bit of my dinner when carrying it on a tray and he whacked the tray, plate, food etc. up out of my hands and it all smashed against the wall and food went everywhere.
When my mother left him, I felt sad and worried for him, but I always knew it was the best thing for her to do and felt she should have done it sooner.
I've since studied psychology and when learning about childhood trauma, they talked about how unpredictable trauma is the worst and can have very deep effects, because of never being able to full feel safe and off your guard. They explained that for example knowing that x person 'always kicks off' when they've had a drink, brings some predictability and forewarning in and can at times allow for protective factors to be put in place. However not ever knowing what will be a trigger means no downtime from it. This was a huge lightbulb moment for me, the day we had that lesson.