Snape had seen Harry and herself shortly before saying the deadly words, signalising to stay quiet so they wouldn’t get caught by Death Eaters. But now, it gave the impression that he was not a spy but a true Death Eater since he had killed the headmaster, but he hadn’t killed them, so he couldn’t be a real one, now could he? Why did he protect Harry from Death Eaters if he followed the insane person who wanted Harry dead?