User blog comment:Molly weasley5678/why did snape die/@comment-5238676-20150602140516

There was also the bit with the Elder Wand. Voldemort felt himself unable to adequately control the Elder Wand, because he was not the one who killed its previous master (Dumbledore). Snape was the one who killed Dumbledore, so Voldemort figured that him killing Snape would transfer the alligence of the Elder Wand to him.

The problem was, Snape and Dumbledore planned the murder earlier, hoping that the Elder Wand would end up without a master. Draco comes along and disarms Dumbledore beforehand, making him the master of the Elder Wand. When Harry disarmed Draco at Malfoy Manor, the Elder Wand transfered its alligence to Harry.