Snape is acting stupid because that's basically the only way he can avoid the obvious route: just breaking down and telling Harry the truth. But little by little, his other "options" are being stripped away and he'll have to climb that wall.

That's an interesting perspective; I probably didn't see it because I tend to sympathize most with Harry. Harry doesn't see it in those terms because, as he sees it, his starvation or not-starvation doesn't impact Draco and Snape survival in the same way that critical information about Voldemort impacts his own survival. So long as he kills Voldemort, then they'll be as safe as anyone else.