I was talking about this with my wife tonight. All the people saying that bandits are great, because they'd act like that in real life are full of crap.
When societies break down, you don't have everyone running around slaying each other like a big free-for-all deathmatch. Armed robberies and the occasional murders, sure, but not the kind of shoot on sight attitude that a lot of players have on DayZ. ( I would do it simply because I refused to be taken by surprise by someone) Generally survivors would seek out other survivors and work together. (or mooch off the others) The people who are already living life like bandits would take advantage of it, but it wouldn't instantly turn the majority of people into them.
Look at New Orleans after Hurricane Katrina. Mass lootings were common, with sporadic rapes and shootings thrown in. (remember that New Orleans on a good day is one of the "Murder Capitols" of America) It was pretty rough, but it wasn't all out war between the survivors.
Gamers tend to operate in certain ways.
1. Role-players: These are the guys who will take the mod seriously, and actually play the game pretending it was really them in a real-life zompocalypse. (this can include the casual players too) They're the ones who will prefer to work together with other people to survive and scavenge better stuff. They generally only fight other players when forced to. Occasionally they will shoot on sight.
2. Power-gamers: These are the guys who will spawn in and immediately make a bee-line for the best weapon spawns, which they have memorized or mapped out. They'll kill you to acquire better weapons or food. Anything to increase their own power. I think a lot of clans would fit in this group, or possibly an even mix with the role-players.
3. Griefers/Player-killers: Pure-bred bandits. They're only playing to grief the people taking the game seriously, or just have more fun killing human players rather than zombies. These are the ones who will kill you on sight, or backstab you at the most opportune moment.
I downloaded that singleplayer version someone talked about earlier. I was messing with that while waiting to see what the real devs were going to change in the next patch or two. Basically just reading this thread now and then because the zombie genre interests me, and its interesting reading your posts.