[He's in a bad mood, and whenever the Dark Lord is in a mood, people tend to die. This is something that Snape knows with complete certainty from all the years spent in the dark wizard's service. The Death Eater in him is telling him to leave the wizard as soon as he can, to flee. The spy knows that fleeing is not an option, that he will lose favor should he leave before being dismissed. And the bit of him that belongs to Dumbledore knows that there is a better chance of others surviving should some of that anger direct itself at him.
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So he stays.]
Can I be of assistance?