Why is Narcissa in tears in this scene?

Question

In the scene from Harry Potter and the Half-Blood Prince (2009), where Snape makes an Unbreakable Vow with Narcissa to protect Draco and fulfill his assignment if he fails:

Why is Narcissa in tears in this scene?

Answer 1

In the book she's crying because Snape basically confirms to her that her son is being set up to fail, and probably be killed in his attempt to kill Dumbledore (e.g. that he'll be forced to use lethal force against Draco).

‘The Dark Lord will not be persuaded, and I am not stupid enough to attempt it,’ said Snape flatly. ‘I cannot pretend that the Dark Lord is not angry with Lucius. Lucius was supposed to be in charge. He got himself captured, along with how many others, and failed to retrieve the prophecy into the bargain. Yes, the Dark Lord is angry, Narcissa, very angry indeed.’

‘Then I am right, he has chosen Draco in revenge!’ choked Narcissa. ‘He does not mean him to succeed, he wants him to be killed trying!’

When Snape said nothing, Narcissa seemed to lose what little self-restraint she still possessed. Standing up, she staggered to Snape and seized the front of his robes. Her face close to his, her tears falling on to his chest, she gasped, ‘You could do it. You could do it instead of Draco, Severus. You would succeed, of course you would, and he would reward you beyond all of us –’

Snape caught hold of her wrists and removed her clutching hands. Looking down into her tear-stained face, he said slowly, ‘He intends me to do it in the end, I think. But he is determined that Draco should try first. You see, in the unlikely event that Draco succeeds, I shall be able to remain at Hogwarts a little longer, fulfilling my useful role as spy.’

Much of the preamble to the unbreakable vow is cut out of the film, but I think we can assume her emotionality is for the same reason, that she thinks her son will be harmed.