I was running around the map with two NPCs, then disconnected. When I logged back in half a day later, I saw that another player had killed my NPC, killed him on a PVE server.
Give me back my NPC and his stuff or I’ll quit this game and write a negative review!
Hello and welcome to the forum!
Funcom doesn’t read this forum for customer support. Try to contact their customer service directly - but frankly, don’t expect much from them.
Sad to hear about this event, if that helps. 
It is possible to kill other player’s followers be logging out while standing right next to them and logging in again. There is or at least was the ability to kill followers who were in down but not out. The former was reason enough to move all my thralls indoors and no longer make my bases open to anyone else on PVE servers.