To kill or not to kill: DayZ Friend or Foe

Why do people always ask if you are friendly in DayZ. Today I had to sneak in under the cover of darkness into Electro. While scavenging for supplies in a certain multilevel building in the middle of town I heard footsteps. I asked on direct chat, "friendly" a response came out from the darkness "friendly". Phew, I sauntered out to the hallway to pick up some loot from the stairs . As I was searching I heard a gunshot , Mr Friendly had just fired his revolver at me in the darkness and missed. I ran a level down and hid in a room . Consequently I went onto direct chat and proclaimed " you fucking asshole now I'm gonna have to kill you".
So I waited in the darkness, then I heard the unmistakeable sound of footsteps. Tension filled the air as I clicked a mag into my enfield.
After some moments I stood up and sprinted to where I had been standing when he had shot at me. Sure enough he was there crouching over the same loot. Bang one shot to the head with the rifle and he was dead. Such is the way in DayZ . No mercy for the supposed friendly player.
This whole episode highlights the issue of wether or not to shoot other players on sight or not. You actually do not have time to ask someone if they be friend or foe because you have about 2 seconds to either kill or maybe be killed. Such is life in the Z. I hear on Russian servers everyone shoots everyone on site. Hmmm.
You can't necessarily trust anyone who says they are friendly either. In the viscous fight for survival folks will usually shoot first and ask the questions later. However if you can make a true friend you are lucky. Generally speaking someone who will come onto Skype or team speak is trustworthy. And then there are the times when you must trust others with your life or to rescue you by way of morphine or blood transfusion.