“Dumbledore expected Voldemort might violate his tomb and take the wand. He had hoped that any spell Voldemort cast against Snape with it would fail because the Elder Wand would not kill its master, especially if its master counteracted with purely defensive magic, as Harry did and Snape surely would have.” Harry PotterAlbus DumbledoreSeverus SnapeElder Wand Book:Snape: A Definitive Reading Source: Snape: A Definitive Reading
“Dumbledore wanted the Elder Wand to recognize the mercy and regret in Snape’s Killing Curse and transfer allegiance to Snape, quietly. He knew he could trust Snape to be a good custodian of the Elder Wand, since Snape’s signature magic is purely defensive: when he has to force himself to attack or use Dark Magic, it’s only in order to protect others, then immediately dropped.” Albus DumbledoreSeverus SnapeElder Wand Book:Snape: A Definitive Reading Source: Snape: A Definitive Reading