It's Sam's voice. It's always Sam's voice, but it never really is. Not anymore. Not since his little brother crumbled at his lowest and said 'Yes'. Not since Dean turned his back on his world and let the Devil swoop in and take Dean's universe away because Dean listened to a bunch of stupid angels.

So yes, it's Sam's voice, except it never is. It's Lucifer's and Lucifer may be pulling Sam's strings but Sam's voice was never so cold nor so charming. Lucifer speaks like a snake, like the Prince of Lies that he is, and Sam always spoke with his emotions bleeding through and saturating the words, even when he was trying to wear a mask and pretend he was fine. Sam could never fool Dean and neither can Lucifer. The Devil may pull Sam's strings and pretend to be his brother but Sam was Dean's world, Dean's everything and every little aspect of him was forever imprinted in Dean's brain and Lucifer pulled the strings all wrong.

It was pathetic really that Dean didn't do anything to stop it, but he'd tried all he could and killing the Devil would be killing Sam. Then again maybe Sam was already dead and Dean was just clutching onto the shell of what was left with all the terrible disgusting beauty of insanity.

Lucifer kept him locked up like a pet and when he looked at Dean with eyes that were Sam's but not Dean obeyed his every whim because Sam was Dean's world, his everything and Dean was just as much of a shell now as Sam and the Devil was pulling both of their strings, just in different ways.