The thing is that hitting is not "always" wrong. I want my children to learn to defend themselves. I will send them both to decent martial arts classes where they will learn when it is and (mainly) when it is not OK to hit. We are not yet a post physical society.
I have virtually given up smacking (and my smacks are now so ineffectual as hardly being worthy of the name) due to feeling bad about it. I don't think I am a better parent for it, possibly worse, as I have lost (some) confidence in my own instincts.
However when my children shock me I still can shout loudly enough at them to cause upset and, yes, I do do it in anger. This is mainly when they hurt one another or me or my wife. They need to learn certain things are not OK and big scary daddy will emerge if these things happen (casually poking in the eye is the latest). I am still amazed at threads on here where parents permit their children to bite them (for instance) without any kind of physical response. They are literally standing there with teeth penetrating their skin counting to five or threatening a time out. To me, there is something wrong with that. Children need to see their parents as real people, not ideal people, and learn that extreme behaviour carries some personal risk to them. I unashamedly smacked my children if they bit me (and still would).
As for the idea of reciprocity ("Daddy hits so I can hit") I am all for it. I tell them that they got a smack for biting me and if I or anyone else bites them they are welcome to smack and that is fair....but I don't and never will bite them because I am not an animal. They get it, even the two year old.
Disclaimers to the rabid antismackers and antidisciplinarians:
1/ The vast majority of the time my children behave well and have no need for shouting or any other kind of discipline.
2/ Yes, we as parents use lots of other strategies including reward charts, distraction, explanations, trying to get them to empathise, time outs, toy removal etc etc
3/ They are both reasonably gentle children and are well liked by their peers and at nursery.