Dawn watched the exchange with wide eyes, Ben's begging was nothing new she'd seen it even earlier today (was it today, the hours just seemed to bleed together in a haze of pain and worry). And then he had been lying, was he lying now? Did it really matter? There was no way he could really control Glory, how could he. And worse, what if he was lying again, was if he was just as bad as the thing inside of him.
She hated him so much at that moment for showing his stupid face, for making Buffy have to make this choice. It wasn't fair, it wasn't supposed to end badly for her but even though she wasn't jumping off the tower this time it still was.
"He's lying, Buffy," she managed to croak out in their exchange, trying to raise her voice above the sounds of battle before them. "This is how he got me, tried to play the poor Ben card, and then he let Glory out," so maybe she was simplifying the meeting right then but Buffy needed to be able to do this with a clear concious, and if that meant Dawn lying a little, pretending to be one hundred percent sure Ben was just as bad as Glory then so be it.
And she forced herself to watch Buffy lift the sword for the killing shot.