Great questions OP.
I was smacked as a child. Pretty much just by my mother and I remember clearly being told that if I didn't stop crying that she would give me something to cry about. I remember feeling so frustrated and angry that I would bite myself and would be hoping that the smack left a mark so that I could make her feel guilty about it. I've been slapped in the face, hit with a fly swatter and a leather strap. My mother didn't have a line and continued to hit me in frustration into my twenties because she couldn't 'control' me. At which point I told her that if she ever hit me again I would hit her back. I hated it and I think it taught me to hit when I didn't get my own way. My mum still does it to my dad, he would never hit her back. I hit my first boyfriend in frustration and he told me that if I ever hit him again that he would hit me back. I thought that was fair enough and it was pretty clear from then on that I needed to change this learned behaviour. I find now that I still raise my voice and have aggressive feelings with my DS in frustration and I utterly hate myself for it. I don't smack. My ultimate goal is to never raise my voice and definitely never hit my child. I have not been perfect but I put a lot of effort into learning about gentle parenting methods so that I don't follow the same path with my own children. What were the questions again? lol I'm on my phone and can't go back so I'll just say that I don't think physical violence is appropriate EVER regardless of age or circumstance. I don't expect another person (adult or child) to hit me so I wouldn't do it to anyone else for any reason.
As you can see, smacking did impact on me in a very negative way. FTR I love my mum, she is a wonderful mother and I don't think she is a bad person, I just don't agree with her parenting choices.