If the player has not been and is not being involved in any kind of combat during some time (let's say 10 seconds or 20 seconds) I see no reason why the player cannot just be logged off and be allowed to log in alive again later.
On the other hand, if the character is involved in any kind of combat (actively or passively) during that grace period, the character should either die immediately or receive attacks as long as someone keeps hitting him or until he dies (whichever comes first), The reason for that is that it prevents many types of exploits.
Hitting a boss, kiting it out of its zone and disconnecting is a well-known exploit technique in some games (screwing up the badly implemented AI so the mob won't move or fight back). Hitting and logging off may also be a very effective form of griefing in PvP. Or, a dozen other things you could come up with.
If disconnecting means certain death, none of that will work.