Munakata hadn't forgotten Mikoto or the position Mikoto had willingly placed him in, forcing him to play executioner with little regret. He didn't doubt his Lieutenant, never for a moment, but Munakata found himself reluctant to force her to slay him the way he had Mikoto.
A backup plan, one she could hardly miss, a blatant admission of his own weakness, but Awashima Seri shared his particular weakness, the inability to walk away from their own duty.
This wasn't her duty. He'd given it to another. Even so, he should have known, she would do what needed to be done.