@XL thats not a spoiler, it probably did work as it weakened miyabi enough to be locked in that shrine. The problem was the he probably didnt have the ability to kill miyabi since he was numb from the bite or he was simply afraid that he wouldnt actually be able to kill miyabi so he simply locked him up.
If only Igarashi could be alive long enough for the rest of us to admire. Hell, he was probably the most interesting character we've had in a long while.