What I like about the "fake persona" that O!Ciel puts on when he goes on a mission undercover is that what we're looking at didn't use to be fake. He used to be a gentle and caring kid until trauma took him over, but deep down I believe there's still remnants of that side of him.
maybe i just love seeing sebastian change his perfectly beautiful appearance into something so grotesque and inhuman because it reminds me of my favorite kuro genre which is HORROR..... but also because it reminds me that yes. that really is one HELL of a butler!!