He can be killed. The question is can the Destiny Universe kill him. Is he following the destiny rules or his own universe? The OP didn't specify? Even so he still would be extremely powerful. He could wipe out the very source of the Gaurdians instantly seeing that he is "pure" the traveler wouldn't see it coming. As of now, there is nothing that would stand a chance. The destiny game doesn't even follow the grimoire aspects as they should otherwise it might be a challenge instead of a shut out. Even if he is mortal, Goku was designed to be over powered. As of yet, nothing in this game that has been faced, can beat him. Might as well nerf him now.