Killua had finally managed to sneak into Leorio's office and find where he keeps his candy. Usually he hides it after appointments after he found Killua in the middle of going through the bowl and stealing all his candy. But today Leorio forgot. Most likely because Leorio had to give Kalluto a checkup and Mom was giving Leorio a bad time due to her paranoia. How she saw Leorio, the nicest and most kind person in this house, as a threat Killua would never understand. But, he had other things to worry about. Namely, candy.
He grabbed as much as his tiny 5-year-old arms could carry and stuffed more in his pockets. And then walked out the door like nothing happened, hoping that Leorio would come back to his office and see Killua walking down the hallway with his pockets bulging and arms overflowing with candy. On the way, he ran into Milluki.
"Ooo, give me some, Kill!"
Milluki must have guessed from the way that Killua was looking at him like he was crazy that Killua wasn't going to give him any. So he proposed a bet. If Killua could fit 20 pieces of candy down his throat Killua could keep all of it. Now, Killua being the young and dumb five-year-old he was, quickly accepted that challenge. As long as he didn't have to share, he would do anything.
He quickly shoved 20 pieces of candy in his throat and just as quickly, started choking.
Somewhere else on the premises, Leorio's 'Killua is doing something stupid' sense activated, and he quickly went to his office to gather the supplies his would most likely need to help Killua. What he didn't expected was to find Killua choking with a bunch of candy at his feet while Milluki watch from the sidelines and laughed maniacally. He never thought Killua could have this little sense of self-preservation.
After five minutes of doing the Heimlich maneuver on Killua and launching into a lecture about choking hazards and food safety, along with sending Milluki to his room for almost causing Killua's death, Leorio headed back to his office. Killua waited for Leorio to come back with some pain killers but after thirty minutes nothing happened. He walked in and before he could say anything, Leorio already answered the question that was on Killua's mind.
"Yeah, I'm not giving you pain killers. You wouldn't be in pain if you hadn't decided to be stupid."
The shock was clear on Killua's face. And when Leorio flashed him an evil grin in response to said face, the only thing he could think was: 'This sadistic old man!'