[He's being careful!! Though now he can't move with his wrist in Izuku's grasp, but his nervousness about the knife mixed with his flustered reaction just adds to how funny Katsuki finds all of it. He can't help but continue to laugh, so he has to bring his other hand to cover his mouth and smother it.]
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You're-- you're too easy to rile up, Deku.
[Teasing Izuku is his favourite past time.]