I know that is for having a serious plot but MC letting milf do whatver with her evil plans until the last minute is iretarded, because with some he doesn't know what she is plannning, what if this sealed beast had a plague power and infected people just by walking? MC couldn't heal a illness.
The worst part is he knows where she is and what she is doing, that collar have no use if he doesn't use it, had he made it transparent she would probably have been less burdened by it, is the humiliation that pushed her to the brink, really, MC is the one that made things go bad with a half assed punishment he didn't follow through.
I know he doesn't want to kill a human, even more his rebirth mother, but come the fuck on, bitch is deranged and was fine with killing infants or kids that threatened her position as the strongest, at the very least seal her ass.