Why didn't Voldemort kill Harry when Dumbledore was killed and Harry was alone in front of Severus?

I mean, after Dumbledore was killed and Harry was running behind Severus saying "Fight back you coward, FIGHT BACK!" Why didn't Voldemort just come and kill Harry then only. Severus and the death eaters would have holded Harry until You-Know-Who appeared. I didn't get it!