me and my friend started building a base. On one side where the door was, a wooden wall was built, meaning that no one could get in through this door.
When we wanted to score a goal on the other side, we heard noises from my other player. This player was apparently ill.

I was in the house down the stairs, my friend was outside and placed the wall on the fence so he couldn't get in.
I went up the stairs in the house and suddenly another player killed me.
I thought he joined the game because he couldn't get in on the other side, which wouldn't have been a problem for me.
My friend tried to take down the fence to fight the stranger, but he was no longer in the house. The enemy was probably invisible or had logged in and out, no idea, but my friend died immediately without hearing anything.

I find it really disgusting that players like this take away the fun of playing Dayz.

In my opinion this is a hacker!