Mortal Kombat: Armageddon/WMG

The reason certain characters came back from the dead.
In Shang Tsung's bio for the game, it explicitly states that all those who pledge loyalty to Shao Kahn will be soul-bound to serve him, so if they die, they will return to him sooner or later. This can therefore explain why characters such as Sheeva and Motaro had returned despite their deaths being described in other titles such as Mortal Kombat: Deadly Alliance.
 * As for Mavado and Hsu Hao, what's to say Daegon didn't find them and use his abilities as the son of a God to heal them both? After all, Mavado thought he killed off Kabal, and Havik managed to do just that.