Following a discussion of their preparations for the conclusion of the Holy Grail War, Kirei remembers murdering Tokiomi Tohsaka in his study by literally stabbing him in the back. Gilgamesh appears and they form a contract of Master and Servant over his former Master's dead body. Afterwards, they celebrate their victory for a short time, with wine and... other things*.
A snake, looped around his right shoulder and traveling up the side of his neck, to just about his ear—as though it is whispering into it. The design of the snake undeniably calls associations with the Devil or Satan to mind.
Kirei remembers himself as a young boy, being praised by his father in front of his fellow members of the Church for being such a good and respectful son. He's "beautiful", his father, Risei, specifically mentions, hence his name "Kirei". Later on that night, alone in the darkness of his room, Kirei suffers with feelings of deep guilt. He wonders why his father calls him beautiful, if he himself believes he's anything but—to begin with, whereas other people find particular things beautiful, his definition is different.
positive
BETRAYAL: The first Master dies; a contract is formed
*whatever vic will let me get away with
mark
neutral
KIREI: The child realizes something is wrong
negative