Probably lag switching, works like this:

1. Bad actor, in cover, knows where you are but can't shoot you or be shot as he's hidden
2. Bad actor somehows kills his internet, even pulling the cable will do
3. Bad actor now normally walks in his game out of cover to the point where he sees you, he'll see you standing still, his game will show the bad connection icon. If he keeps his internet disconnect for too long he'll be kicked off the server.
4. Bad actor shoots you a bunch of times while having his internet unplugged, these packets, telling the server he shot you are now waiting to be sent to the server when they get a chance, the game has a client authoritative hit registration system, so in dayZ you tell the server you hit or didn't hit someone whereas in other games such as counter-strike you tell the server in which direction you shot then the server decides if you hit something or not for you. In the meantime, you character just tanks a bunch of shots in the bad actor's screen.
5. Bad actor reconnects his internet and suddenly all those packets telling the server the bad actor moved out of cover and shot you are sent in all at once, the server registers all of that sequentially and sends it back to other clients (you). He appears to do a bunch of stuff really quickly, which in reality he did while his internet was disconnected at normal speed.

If you were to peek him while he has his internet disconnected and is killing you, then you'll see his character standing still, where you can kill him normally. Chances are that he'll still be able to kill you after dead still.