In the latest episode of the series, Mitsuki attacked the Fabrications’ leader Ku and revealed that he was really working on the side of the Hidden Leaf all along and Sekiei was hurt by this. Sekiei has been the Fabrication closest to Mitsuki, even vouching for him when he figured out that Mitsuki was probably going to betray them.
But this is a whole new case altogether. Sekiei revealed to Mitsuki how much he had been hurting after losing two of his friends, and now another that he’s confided in him has betrayed him. Sekiei has been the one closest to outright developing “human” emotions, so it’s no surprise to see him lashing out as emotionally as he does in the next episode.
The preview for the episode teases that Sekiei’s battle with Mitsuki will continue to tear his body apart much like his comrades, so Mitsuki still wants to reach out and save him while there’s still a chance. Sekiei’s lost in his emotions, however, so he doesn’t want to listen at all. With Mitsuki still wondering about what constitutes one’s will, the fight here may bring him as close to the answer as he would hope.
Being a created person himself, Mitsuki does not really have a truly emotional side so seeing Sekiei act on in rage in this way may solve some riddles about Mitsuki has about himself. He knows that simply having a heart wouldn’t make these Fabrications human, so now it’s up to him to teach Sekiei the same.