[Sasuke accepts the key without a word, looking totally unsurprised at this revelation. Once in hand, his grip immediately shifts to hold for what it is - a knife, meant for stabbing, rather than something as innocent as a key.

Though he looks calm, his body is tense. It would not be wise to try getting close to him right now, not when he's ready to move the instant anyone comes into range.]

...

...You say that the person who should be sacrificed is "the most evil" one. But...

Shouldn't it be "the one who doesn't want to go back?"

[Seems like the fairest option to him.]