[bowener]

This would make DayZ head and shoulders above any game out there currently and for the foreseeable future. It would be amazing.

Reddit Context
The topic is how to give life value in the game.

It needs to be done through character progression.
Tougher immune system, beards, scars, becoming more fit if we keep ourselves healthy.
However, people are looking at this the wrong way. No matter how valuable a life becomes, how much more it'll be worth to leave someone alive than gun them down, KOS will never be a thing of the past.
You might be asking "Why?" Because it's a virtual space, with no repercussion to taking a life. You won't have nightmares, you won't throw up, you won't shake, it won't weigh on your mind to the point where you'll most likely commit suicide.
At the end of the day, DayZ is a video game, not as arcade-y as most, but it's a video game. No matter what end game, mechanic, etc is put in place, people will murder because "Hey, it's a vijeo gem end ve ken lol".
So, with that introduction out of the way;
Mental Health.
Our actions, our comfort level, the food we eat, player interactions all need to have an effect on our characters' mind.
BUT CYBORGMOUS3, DAT JAST FURCZ KERBER ETTUD
No. It's another, authentic representation of the human struggle. Get shot? Remove bullet, patch up wound.
Become depressed? Take pills, run around a sunny field, pick flowers.
Taking a life is hard. No matter what kind of trained, rugged soldier you are, it weighs on you. Soldiers have regular therapy to deal with the effects of murder.
Overtime, as a KOS'er guns down fresh spawns and vets alike, their mind will crack. First subtly.
Slumped posture, where the back is bent forward, head held low. Subconsciously indicating the character's mental health is degrading.
Then, as the bloodlust takes over and dozens more end at the player's hands, the mental degradation becomes more obvious.
Twitching head, indicated by a constantly bobbing camera, random sound effects only said player can hear. Foot steps, whispers, bangs. In short, insanity.
Naturally, the effects could be countered up to a certain level. Wear warm, comfortable clothes, eat cooked food, spend a few hours laying in the sun, getting comfy. So on.
But after months of butchering, the process would become irreversible. The character would be doomed to total insanity.
On the flip side of the coin, we'd have people working together because of this system. Healing wounded / sick players would improve their mental status. Eventually, fixing the broken items of other players. Weapons, clothes, vehicles. Being constructive.
Trying to rekindle civilization in this bleak world would help these survivors stay sane, even when occasionally, they'd have to defend themselves by taking a life.
Staying sane would have no effect on game play. Simply, we'd remain human. We'd hold onto our morals as everything else degrades around us. The reward would be that, against all odds, we didn't compromise.
TL;DR:
Constant massacre and butchery needs to have a game changing, negative effect on players, so that being helpful can be a reward in its own right.
People won't work together, ever, because it's a video game. No matter how much you want to imply they should. A line needs to be drawn and the developers needs to take a stand on either side of it.

Phenomenal answer.