@WhimsiCat
sadist's entire advice to setsu (chapter 45~this chapter) is basically to defeat seigi in a sadistic way; make her surrender, use gloves to slap her anus (shown earlier in chapter 28~29), she even explained her mentality to setsu as 'funny way of expressing my love' (sadism) in chapter 45, so setsu's actions and mentality this chapter is basically the result of sadist's encouragement and influence.