Seraph by Lily Mayne

I’ve already mentally prepared myself to kill the screaming, murderous monster in the cage outside our camp.
I’m used to it. It’s what I do here. Kill monsters and other people. And this monster, Seraph, is particularly unstable—has been since the moment he arrived from Collector Mary’s monster menagerie to the west, where the military stashed him away.
He’s in pain. I can see it. But he’s not a mindless beast. Not like some of the others think.
When I start guarding him in his cage every night, I begin to see the person beneath the agony that tortures his every waking moment. I begin to see who Seraph could have been if he hadn’t been twisted into this monstrous creature.
I realise that I wouldn’t be able to bring myself to kill him, even for his own sake, but it doesn’t matter anyway. He’s already dying, and there’s nothing I can do to stop it. Not even when new visitors at our raider camp bring with them secrets about Seraph. Secrets the military has tried to hide.
Secrets that reveal the true extent of the horror he’s been put through.
And now… Now, I will kill anyone who tries to hurt him again.