Yes, it's another Ao no/Blue Exorcist drabble. (I actually deleted one in an earlier post because I think it needs more polishing.) This one is an attempt to explain Rin's reaction to the zombies in chapters 55 and 56 of the manga.
Humanity
When he hesitated to destroy the all-too-human zombies, Rin knew the others didn’t understand. He didn’t want them to understand.
They were human and would always be human. They didn’t have to hold their humanity in a white-knuckled grip while all the world tried to tear it away.
And, he was glad of that.
But, it didn’t work that way for him. He was a monster and had always been a monster, just now everyone could see it. His only humanity was in what he did. And, in what he didn’t do.
Monsters killed humans.
And so, he could not.