I would never disagree with the statement that fighting can be healing and empowering, I believe it can be just as much antidote to as it can be amplifier of depressive tendencies. Your point about it being a double-edged sword seems to capture it all. Life is a fight, fighters are the artists of life par excellence, and so it follows that they will experience happiness in its fullest aspect just as much as they are at risk of depression. I do however believe that the hunt, although undoubtedly dangerous, is fundamentally different than fighting - most importantly the pack aspect, the asymmetry of hunter-prey (whereas fighting is hunter-hunter), the lack of crowd (I suppose you could argue that the crowd waits for food at home, but they are not immediate witnesses to either success or failure as in fighting) and the difference in preparation (the grueling grind of the fighter vs. the non-training of the hunter) towards the event. I'm sure we've always fought, but I blieve it was likely more a matter of manifestation of power (dominance) than application of killing efficiency, as you would see in a fight betweens animals over mating rights for example. I'm very convinced that the life of fighters is very different than the evolutionary ontology of human beings in a hunter-gather context.