In the book "The Soul of the Sword" the author points out two types of fighting in nature: Inter-species fighting, and Intra-species fighting.
"Inter-Species" is the stuff where a cheetah stalks and hunts his prey, and the prey does what it takes to survive. No rules in the animal kingdom, and no ego to defend. Humans are one of the rare species that prey on each other. To me this the same as Darth's "Combat". You do what needs to be done, and walking away, or diffusing isn't a choice. (Taking the nike defense, on the other hand, is very common.)
If we have the opportunity to prevent one human from preying on another, weaker human, we have the duty (nods to Darth) to prevent that. We have to remember that we are working in a system that has very few (if any) rules, and that those rules are probably not the same for all parties involved.
"Intra-Species" is fighting for dominance with others of the same species. Deer fight over a doe, snakes wrestle for dominance, rams, well, ram. These fights are for "rank" and pride. They are usually between evenly-matched foes, killing moves are not allowed (Deer won't use their hooves, ratttlesnakes won't bite, horses wont' kick, etc), and it's usually for the benefit of an audience. (Females, usually) To me, this is the same as Darth's "Duels", or most bar fights. It's a challenge that is issued and accepted, usually to stroke someone's ego. This type of fighting is usually illegal, and for me personally, immoral and irresponsible, unless done in a sporting environment (which is primarily for training and testing, rather than conflict).
I pretty much sum all fighting by those two models. There are wars and battles that follow the interspecies model (defense), and there are some that follow the intraspecies model (dueling). Some people do competitions for training on how to defend, and some are in it for ego.