I was in a vanilla server (no mods), and PvP was turned off. I was Wendy, and I was basing with a Wilson. We were out killing spiders, and I hear Wilson's death sound, and a death pop-up reading "________ was killed by Abigail! He became a spooky ghost!" I forgot the Wilson's name. As soon as the message popped up, everyone on the server got confused in the chat.
Why did this happen? I made sure that the server I joined had PvP off.
Note: I did not see what Wilson was doing before he was killed, because we were in a forest, and he was obstructed by trees.
Also, I just spawned Abigail a little under a minute before she killed Wilson, if that information is useful.
Wilson was obscured on my screen, so I'm not sure how he got Abigail to agro. onto him, sadly.