NPCs have a variety of behaviours. While you can be reasonably sure that mutants are largely aggressive and see you as little more than food, the other humans in The Zone are far more complex.
The military have a job to do and are good at it. If you are in the Zone, you are a target so if you pick a fight with the military you'll have just enough time to appreciate the dynamic Combat AI and it's intelligent use of cover before they fill you with bullets.
Other Stalkers however are a different proposition. Stalkers are not there as enemies until you make an enemy of them. You can choose the aggressive path and make enemies of individuals and even Stalker clans. However if you consider your approach carefully you will uncover an in game economy based on trading items, food, weapons and information.
It is up to you - choose to see Stalkers as enemies or as a means to aid you progress.
There are also a number of "Dealers" in the game. These guys really control the economy within The Zone. These are the guys who can give you jobs, sell you equipment and reveal information about The Zone. Dealers are respected as they provide a fair service to all Stalkers, so if you anger or even kill a Dealer you can expect every Stalker in The Zone to target you.
To survive a Stalker has to trade. Just like in the real world you can buy, sell and exchange everything you need from ammunition to artefacts. The currency is Soviet Roubles and you can do deals with the army, scientists, dealers and other Stalkers.
There is a code of etiquette which you would be well-advised to follow: On meeting a veteran Stalker you first exchange greetings, you agree to remove weapons, then slowly move closer and begin the trade. Violations of this code will elicit a severe response.