There's banditry, and what I'll call raiding. Banditry is when you shoot a man for his beans. Many people do this. Some targets are worth ambushing and killing, some are not. That's just how it goes in a world like DayZ, and I see nothing wrong about it in the context of the game.

Then, there's raiding. I used to be a raider. Being a raider is like being a post-apocalyptic Atilla the Hun. You run into a grocery store with a blazing AK while mic-spamming battlecries into sidechat, not because the people inside have beans, but because you saw them in the grocery store and by God, they're going to die today. Then, after you finish off the zombies and wait for the narcochemical cocktail of serotonin and dopamine from the rush of the kill to clear out of your brain, you check to see if they had anything on them. No beans? No bullets? No map? Hell, who cares? You're a raider! You'll always have more prey!

Did I consider myself a CoD/BF style FPS player? No, not really. I've been playing Arma for a while now, and I'm not generally kill-hungry in most competitive FPS games that I've played recently. But DayZ honestly gives better rewards for killing than the point and class based shooters. I don't want 100 xp and a new gun flashlight. I don't necessarily want that guy's stuff. I want the satisfaction of knowing that I killed a guy who was just as likely to kill me, and that I survived. Fighting in CoD is like playing penny poker. Yeah, you can win and lose, but you don't lose much, and you don't care about losses, and you hardly care about winnings either. DayZ is like playing for much larger sums of cash. The "real" consequences of death in DayZ means that life is precious, and fighting and killing in DayZ means you've gambled in a game where the stakes are far higher and you've won. It's the same reason that people like gambling.

TL;DR: Some people view killing in DayZ as the ultimate game of chance/skill, and derive immense visceral satisfaction from it, nature or history of the game be damned.