I understand why Draco was the mast of the Elder Wand, not Snape, and then how Harry was the master, not Voldemort. But did't Voldemort kill Harry? Does that count? He was dead for a little bit, right, so does't that make Voldemort the true master of the Elder Wand?
No. The text explicitly states (with the authority of the omniscient narrator) that Voldemort never had mastery of the Elder Wand. It was merely in his possession for a while:
Harry saw Voldemort’s green jet meet his own spell, saw the Elder Wand fly high, dark against the sunrise, spinning across the enchanted ceiling like the head of Nagini, spinning through the air towards the master it would not kill, who had come to take full possession of it at last. And Harry, with the unerring skill of the Seeker, caught the wand in his free hand as Voldemort fell backwards, arms splayed, the slit pupils of the scarlet eyes rolling upwards.
As to why he didn't become the master, the answer is that Voldemort has confused a willingness to kill with true power.
First to gain Elder Wand's allegiance you need t defeat previous owner in battle. Harry and Voldemort did not battle since Harry willingly went to die.
Also Elder Wand's core is the tail hair of a Thestral. As described on wiki a Thestral tail hair as a potent yet tricky substance to master that can only be handled by a witch or wizard who is capable of accepting death, since only by such can it be seen.
Voldemort is scared of death and does not accept it so he couldn't be master of death.