Junko, because she's not always in mood to do evil. I mean, she spent a whole year with her classmates and didn't kill anyone until killing game started. Thought, being locked is gonna make her bored and she'll probably torture me for fun. ̶B̶u̶t̶ ̶t̶h̶a̶t̶'̶s̶ ̶c̶o̶s̶t̶ ̶i̶t̶