idk why people didnt like the ending. because its obvious the police organization are not going to public what happens inside. most drama about police is end up like this. the only thing will happen is removing the most corrupt leader. about ren yeah he is heartless but he explained that in the series in order to stop daizen he need to do what hes doing even it means sacrificing others,we can see daizen is much more a demon than ren and about yua she did that because she really loves ren even she knows ren is trying to use her. thats why he always said he sold his soul to a devil.

"He who fights monsters should see to it that he himself does not become a monster. And if you gaze for long into an abyss, the abyss gazes also into you."