Someone who is immortal cannot die. In other words if you don't find a way to destroy them, they will live forever. Like Morro for instance. After he died, he couldn't die again so he would live forever, but since water can "destroy" ghosts, he was destroyed. There is a difference between destroy and deceased. Deceased just means that you're mortal body has died, Destroyed means that every part of you (Including you're spirit) has been destroyed.