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I know that the answer to that question is to obtain the Elder Wand, but I think that approach doesn't really make sense.
We know that death is not necessary for the Elder Wand to change hands (Grindelwald stole it from Gregorovitch, and Albus obtained by beating Grindelwald in a non fatal duel) which means we know Voldemort didn't have to kill Snape to obtain the wand, that was just one way of doing it.
Snape was a powerful wizard(maybe one of the strongest of the death eaters) and had proven himself more useful than almost any Death Eater(providing years of information on Dumbledore and later killing him), Voldemort probably didn't want to kill him unless it was absolutely necessary.
Couldn't there have been some other way to get the wand from Severus? Like challenging Snape to a non-fatal duel for ownership of the wand, or perhaps even giving Severus the wand and then stealing it, Grindelwald style?