Why are people so hostile?

Seatlle 109 South-Coast Airbase Dude: You're a prick.

I logged in on the bottom floor of the airbase building. I then said on voice, "is anybody here?"

I heard feet scuffling. I looked up, between the opposing sides of the staircase I was under, and saw legs. I could have shot, but didn't.

"I'm friendly, I have a gun but I'm friendly, please don't shoot. I am lowering my gun now ((double tap left-CTRL)). I'm going to leave this building. No use killing me, I have no ammo or loot."

"I'm coming out now, please don't shoot", I said, and with my gun lowered, side-strafed out in plain view of the man. I then Q-E friendly-waddled for him for a few seconds.

Then he shot me in the head.

Seriously? Are some people this socially-retarded? Introducing the global-voice only scheme was meant to humanize players, and voice chat was supposed to make players less likely to slaughter people, seeing them as humans and not NPCs. It's worked a lot, surely, but it blows my mind that there are still basement-dwelling losers who play like this online.