You can either goad a Wight into killing you with its Life Drain and then promptly have someone else kill it, or you can have your dying spirit make a deal with some evil entity. As with creating a Wight, I'm still not sure why you would want to become one. Turning into a Wight warps your view enough that all living, even if they were friends at one point, suddenly become enemies and you feel compelled to kill at first sighting of them.