This is at the time where Killua failed the hunter exam by killing some one and he was chained up to a wall for Mizuki to whip, Milluki had to go so he gave you the whip to punish Killua. Killua had an annoyed expression, but its obvious he was used to this pain, cuz he's an assasin. Milluki: "Make it very painful, okay?" Milluki leaves Killua: "Ugh..."