Maybe we should put the hostile human enemies on a release scale, like how it’s planned with special zombies? But instead of rating it on how dangerous the zombie is, rate it on how reasonable the human is.
The first starting enemies would be present from the start, and are simply desperate people just trying to survive by looting, just like you. They think of a player as a player thinks of current NPCs: either a psychopath who will shoot them and their family down for no real reason, a crazy person who makes a snack from their own lighter, or just an asshole in general. However, they won’t attack you because they’re still basically human. If you try acting dangerously towards them or be sufficiently cybernetic/mutated, they run off. However, they present a problem in that they go for essential supplies you want to have since they’re scavengers just like you.
The second enemies would be around after a week and would be criminal scumbags who will take what they need from anyone who they can take it from. These guys are not like the raiders from the Fallout 3 series: they are simply people who were assholes in regular life but are now given guns and a lawless situation to do what they like. They act similar to wolves and other monsters with behavior specifics: they are stay away from armed players, players who just killed a human, and anyone lacking in valuables. However, if the player is not any of these three things, then they go in for the kill. So, in essence, they are not psychotics but they are criminals and don’t hestitate in murdering you.
The third enemies would be around after a season and would be creepy survivalists who will kill you just for being on their turf. They have seen some serious shit, and know that just because something looks human doesn’t mean it is, so they don’t trust anyone anymore. They will murder you, and are also very good at sneaking around since they wear ghille suits that require a perception roll to detect. Thanks to their paranoid attitude, everyone who goes for them is a kill. Luckily, they tend to be self-sufficient and put up warning signs of mutilated corpses around their territory, so they won’t bother you if you don’t bother them.
The forth enemies would be around after a year and would be psychotics who have finally snapped from all the insanity they have seen from the past year. They can be horrifically mutated, heavily equipped with late-game weapons and armor (Or at least, their attacks/defense would have the same effects as late-game weaponry and armor), and/or bionically modified, making them a tough foe to beat. When they are encountered, they will attack you even if you are a heavily armed mutated cybernetic demon. They are just plain monsters.
So, why are these people attacking you? Well, we have to face that the lore in Cataclysm says that this the worst fucking apocalypse ever. It’s like a horrible combination of Revelations, a zombie movie, and a Lovecraftian incursion. As the situation grows worse and worse, with hope of rescue growing steadily down the tubes, who is to say that the people would not too grow worse and worse? The good people either get killed, leave New England for greener pastures, or lock themselves in their houses. The only people left are the insane, the bandits, and the desperate. By the end of the first year, I imagine it’d be a similar situation to Cormac McCarthy’s The Road: everyone is so desperate and the situation so bleak that attempting to care about your fellow man just seems pointless.