This is my point.
Human nature has an immense need for revolt, for agression, for conflict. Traditionally we resolve these conflicts through violence, and the cost to that can be tragic (namely death).
Through our overruse of violence, and extreme violence we harbour an acceptance of it, and one that is not necessarily needed. There are much safer ways for human kind to sort out our differences. As for the need for agression, there can even be safer methods of fighting, such as boxing and martial arts.
On a political context, we have further moved towards eqaulity, the amount of fights has gone down overrall, as has crime, and I am simply saying that perhaps we are evolving out of the need for violence. (as opposed to news media that claims fights increase, situations of fighting have gone down overrall).
Wether or not there is an acceptale line of violence is a slippery slope, for that can always be stretched, and is almost impossible to enforce. As such we can leave this aside as it is no longer of use to us.
Does human nature have a use for conflict, yes of course, it is what progresses humanity, technology, society and so on. But conflict need not involve violence.
As to your eariler point of fighting not being considered a big deal, it was also less common for a person put through bullying and torture to openly admit mental health issues because of it (partly due to lack of science, and of acceptance of mental health issues). If you have ever been victimized, you will understand this, if you haven't than it would be very hard to grasp.
I'm coming from the perspective of someone put through shit most of my life, as well as being a humanist, a utilitarian, a socialist, and a believer in absurdity of meaning. So there are bound to be fundametal differences in our views.
I guess my points are, the survival instinct of fighting as a means of survival can be adapted into less destructive ways, and that fighting being acceptable only can occur when both sides are willing and rational, something that would be impossible to enforce.
Waiting watching my life I'm wasting my time
I'm losing grip from life
Going back to my dreamspace leaving it all behind
soon the things will be allright when I get there
NS METAL HEADS