It was actually Creed. Logan chopped his head off, he went to Hell and Satan made him his pet. Then after Logan beats the Devil, Sabretooth turned on him. He probably just escaped Hell with Puck and anybody else who made it out after Wolverine created the power vacuum.
And the reason being Satan explained that even if a person died, if their soul was intact they could come back to life if they escaped Hell. However his blade the Soul Sever, Soul Reaper(Something like that) could kill your very soul. So whoever held that had power in hell as they could permanently kill you.