A simple system doing a simple job, if the player dies from another player it change his gamemode to spectator and teleport the player to his killer, after 3 seconds he will be respawned and sent back to the set spawn using the command "/skypvp setspawn", if the player died from any other thing like: Mobs, Void, etc his gamemode will be changed to spectator and he will be teleported to spawn, after 1 second he will be respawned.