Zombie-Loan - Season 1 Episode 5 Sacrifice

When the office investigates a mysterious "butterfly" group, Koyomi and Michiru get mistaken as two of its members due to the butterfly symbols on their shirts. However, they get found out after Michiru tells them how bad it is to kill people, so Yomi comes to Michiru's rescue when they attack her. When Shito, Chika, and Shiba arrive, they follow the leader of the group and kills him, but it is later revealed that the zombie they killed was not the real leader.