So ... in the end they disguise Kiroumaru in order to get him killed by the Messiah. After the disguise falls off and the Messiah realized he just killed a rat the death feedback kicks in because the Messiah thinks he himself is a rat. So he dies.
But if the Messiahs death feedback is locked in on rats why didn't they just cover for Kiroumaru by picking off the surrounding rats and let Kiroumaru kill the Messiah directly? Because the Messiah can't use his Cantus against him. Why wasn't THAT the obvious plan instead of having him sacrifice himself?
The best case scenario with this plan would've been: The surrounding rats get killed by Saki and Satoru with Cantus. The Messiah is helpless against Kiroumaru because his death feedback and attack lock prevents him from harming him. Kiroumaru can effortlessly kill the Messiah.
And EVEN IF the Messiah somehow overrides his attack lock and kills Kiroumaru, he would just die from death feedback since we know at least that works. So basically the outcome we actually got.
It makes no sense that they would jump to the conclusion that Kiroumaru has to sacrifice himself without first considering the obvious fact: The Messiah can't kill him under normal circumstances!
Am I missing something or did the author simply want a tragic martyrdom for Kiroumaru and was willing to sweep a much more obvious solution under the rug for that?