Law can have some influence on ethics, though I think the usual flow of ethical thought is circular. Law, cultural norms, authoritative and peer opinion conspire with our existing temperament and thought to determine our views.
We tend to have predispositions towards a particular view, we then look out into the world and find things to confirm out feelings about the way things "should" be, this reaffirms our views, which begins the cycle again. It seems rare for folks to question their initial premises in a critical way.
I think this pattern is very difficult to avoid. It is something I find particularly frustrating in my own life. I notice myself falling into this trap from time to time which makes me wonder how often I don't notice. I guess it's an epistemic problem as well as an ethical one for me.
I tend to think that the rest of humanity is at least as stupid as I am myself, but this maybe a logical error on my part
To bring it back to your questions however, I think you are right to think that violence is easy to abuse. I hold non-violence as one of the primary Ideals of my personal ethics largely for this reason. (By this I really mean not initializing violence, sometimes it seems necessary in defense.)
Violence, especially when motivated by feelings of righteous anger is very emotionally rewarding, and thus can become seductive. I don't think I'm alone in having felt the profound feeling of Properness that accompanies righteous punitive violence, though I rarely hear people speak of it that way.
I speculate that people need to believe they are acting out of justice and the acknowledgment of the pleasure involved prevents them from maintaining their illusions, so they actively ignore these influences. Motivations aside; it is easy to be lead into disproportionate (that is immoral) action by these emotions, even as we feel we are dispensing justice.
Is it ever "right" to use violence to dissuade a person from an undesirable, but non-violent behavior? I'm sure that it is not! However my certainty of this is emotional primarily. There is a certain irony to using the Law (that is violent coercion) to prevent parental coercion. Degree is obviously important here, a slap is clearly less damaging than a beating, and I think all would agree that some take beating their children to an extent that is morally wrong beyond all reason.
Where these lines of distinction are drawn is inevitably variable. I'm not one to impose my own norms of behavior on others via violence, but I'm in the minority with that opinion. Most people are just fine with doing that. Violence is in fact still endemic, it's just more polite these days. Lawyers are the hired thugs of our modern age.
I have no solutions; but, as you say, the questions themselves and our experience of them is often more useful than a definite answer to any of them.