He can't believe he's doing this to her—to them!
Then he reminds himself, it's what the boy would want: her alive and safe. The poor love-sick fool would do anything to keep her heart beating—anything—even give up his own life (which, he remembers guiltily, might just be the case). And, all things being equal, the kid would agree with him, this he was sure of. After all, it is for the greater good.
She'll be furious, he knows. The stubborn girl won't listen to reason, no matter how sound, and may even try to kill him over this betrayal of her trust. Or at the very least, she'll wish him dead—rightfully so, he thinks.
He also thinks he might do the girl a favor and carry out the gruesome task for her. Because, after all is said and done, he'll have lost the two friends who are so much more then friends—the son and daughter he never had. For a guy like him, who has no family to speak of and can count his friends on one hand, such a loss will be devastating.
But, like with all things painful in this man's life, he decides not to think about it, and fails miserably. Ultimately he turns to liquor, his one and only constant, and drinks the uncertainties away—all the while thinking: it's for the greater good.
A/N: I know that I promised not to work on anything else until I do something about the crap-load of other stories I have in the works, but, ever since I finished Catching Fire I can't stop thinking about Haymitch--I find the surly alcoholic fascinating. Part of me is disappointed that he didn't tell Peeta about the plan, because seriously, that poor boy has been through enough (I have major love for Peeta, so sue me). Obviously his decision not to say a word was a sound one, though I tend to think they'll torture and/or kill Peeta no matter how much he does or does not know. So why not tell him? Of course Katnis could not be told, no matter what. And yeah, I think rescuing her was for the greater good--she's the Mocking Jay. But, I'm glad I'm not Haymitch because I'm sure he struggled with the decision to betray Katnis (and leave Peeta behind).
Edit: gethsemane342 mentioned that I switched tenses in the middle of this one-shot, and then switched back. I just changed it, so now it's right.