Voldemort killed Snape because he (Voldy) believed that Snape was the true master of the Elder Wand, because he defeated Dumbledore. The way how it works with the wands is that one must either kill, disarm or abduct a wand by force in order for the wands "powers" to work.
But Snape didn't defeat Dumbledore. Dumbledore, planned his death with Snape. Draco Malfoy was the true master of the Elder Wand until Harry took Draco's wand.
It's explained in book five, I don't want to spoil your fun.
Voldemort ordered his snake, Nagini, to kill Severus Snape.
Voldemort didn't know Snape had changed sides, Snape was a spy.
No, Snape's soul wasn't damaged, because Dumbledore had told Snape to kill him.
yes she did
cause dumbledor wanted snape to kill him
the reason why Snape killed Dumbledore was for Dumbledore not to protect harry so that Voldemort could proceed to kill him.
Because voldemort thinks that snape is the master of the elder wand but he isn't.
yes, snape kills professor dumbledor with the killing curse in the half-blood prince
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....)
Severus Snape or Draco Malfoy
No, Severus Snape did.