I seriously doubt that, if zombies did overun the world, a full 98% of the people you meet would try to kill you. Humans are social animals and the roughly 1/20 of society that walks about with no soul behind their eyes, no morals, no respect for the law, is also a group prone to doing stupid self-destructive stuff. So most the sociopaths and Antisocial Personalities would die off pretty fast.
Yah raiders and survivalists will exist, but I doubt you would only find 1 out of every 50 people as the sort who dont shoot you first. A lot of people dont like the idea of murder and humans are social animals.
DAYZ falls victim to the GIFT hard and combines this with the fact that, while you may lose your gear, you dont stay dead. You respawn. And the griefer respawns too, so he can keep trying until he evantually gets his gun and ruins everyone elses attempt to survive.
While it is pretty neat, you might as well remove the zombies and just let people loose in the ruins of a city to kill each other. Once you get good the zombies arnt much of a challenge (and many players have quite as a result of being bored) so either you start player-killing or you sit around by yourself at the edge of the map forever with your gear. A good example of how this is more of a "griefing simulator" is the guns. Pinpointing a gunshot echoing about a city to a single building might be a bit hard, and to a single room in that building all but impossible to the naked ear, but zombies in DAYZ have perfect hearing, so your better off avoiding shooting at them if it is at all possible. So why the guns (and ammo, which after society collapses should get scarce quick) with all their many military variations? For griefers to shoot you with.
Its not bad, but it isnt exactly survival horror or even a very inspired zombie apocolypse scenario. Its a gritty very wide ranging free-for-all shooter deathmatch.