He wants to kill him so he could be the "master of the elder wand"
(except for the fact that Snape was not ever the master of the elder wand....)
Voldemort ordered his snake, Nagini, to kill Severus Snape.
No, he had his snake Nagini poison him.
=Voldemort Orders Nagini To Kill Him=
Nagini only killed Severus because Voldemort told the snake to "kill", so Nagini attacked him.
Voldemort, obsessed with becoming the master of the Elder Wand, ordered Nagini, his serpent, to kill Snape who he (Voldemort) thought was the master of the wand. Because the Elder Wand gives his allegiance to the one who killed its master, Voldemort thought he had to kill Snape to become the true master.
Voldemort didn't know Snape had changed sides, Snape was a spy.
Yes. Well, actually, technically, Nagini (Voldemort's giant snake) killed Snape. But Voldemort told the snake to do it.
No, Voldemort's pet Snake Nagini does.
The snake, Nagini, was floating in a protective shield in the room at the Shrieking Shack that Voldemort and Snape were in. Since Voldemort believe that Snape alone held the power to wield the Elder Wand since he killed Dumbledore, he broke the protective charm around Nagini and sent her lunging for Snape. Nagini bit Snape on the neck, causing him to bleed to death.
Yes, Severus Snape died in Dethly Hallows. Voldemort realized that he wasn't the true owner of the Elder Wand because Snape killed its previous owner (Albus Percival Wulfrock Brian Dumbledore). Therefore, Voldemort had Nagini bite Snape on the neck, killing him.Source: http://www.sparknotes.com/lit/potter7 and my own knowledge of the books
Severus Snape is named the new Hogwarts Headmaster. He continues to pretend to be on Voldemort's side and is always consulting Dumbledore's portrait, he suggests the idea of the seven Potters to Mundugus and only tells part of the plan to Voldemort. He also gives Harry the sword so he can destroy the Horcruxes. Voldemort thought that because Severus killed Voldemort, that Severus was the owner of the Elder wand so he has Nagini (his snake) kill him. As Snape is dying he gives Harry's his memories which helps to fill in the gaps of the story.
Severus Snape asked Lord Voldemort to spare Lily Potter because he loved Lily and didn't want her to die. Dumbledore was disgusted that he didn't care about James or Harry.