Similar situation happened to me recently. I was doing a supply run in Elektro and got aggroed by a zombie while in the pub. I ran into the office building and stationed at the top of the stairs with my Enfield. I had plenty of ammo so i managed to kill them off pretty easily. Just as I started to think I was in the clear, another survivor comes running up the stairs. I frantically yelled over mic that I was friendly, and he typed back the same. He was bleeding and low on ammo, so I decided to be a nice fella and help him get out of it alive. Still at the top of the stairs, another wave of zombies came in, and I cleared them out with ease. Feeling good about what I had done, I slowly escorted him down the stairs and planned to even help him out of the city. However, as we got to the doors of the office building, the fucker puts one right in the back of my skull. He must have been saving it just for me. It's nice to see that there are good people out there. I wish my story worked out like yours. :(