Honestly it seems to me that monster-Hiraizumi, while certainly amoral in the sense that doesn't know human morality, doesn't feels evil. While she did replace Hirazumi it's not clear if it was her own target or if it was forced inside her by the monster teacher. The replacement itself isn't much clear. Did the monster insert into Hirazumi when she was killed or was already present inside her? From what we were shown it seems like the monster was already inside. Were the images we saw when her neck was snapped a flashack or a death delirium?
I'm also not sure that Itou conclusions of the dilemma was the correct one. I wouldn't have any doubts if the replacing monster was the murderer. But like this? I'm not sure (and I think that if monster-Hiraizumi deserved death or not depended on more factors that what we were explained ). IT do imho seems more premature to end monster-Hiraizumi when then main culprit and murderer is still free to do what it want.