Hi Carla! I think your theory is probably the most logical one, possibly even the most likely, and I would be 100% on the bandwagon if JK hadn't spent so much time harping on how murder splits the soul. I just can't see how that would fit with DD wanting Snape to kill him, Unbreakable Vow or not. If he had mentioned anything about being able to heal such a soul-split, or murder not splitting your soul if you do it for the right reasons- anything at all that would mitigate the circumstances- but he didn't. That's the biggest reason why I don't think that Harry is going to kill Voldemort with the AK- or do anything to him that could be construed as "murder."