@boogle
Well I don't disagree with you. After all, that child was willing to murder and there's no telling whether he is lying or not.
That said, there are several reasons the MC decided to give him a chance:
1) since he was liked by fairies, he couldn't be an inherently bad person
2) he is a naive child that was tricked into accomplishing this and once the King exposed the trickery, he gave up and revealed everything about himself
3) he is loyal - he was captured, had failed his mission and yet was willing to stay silent to protect the one who had hired him even though there was nothing to be gained from it and instead there was a risk to be tortured - a wilier person would have tried bartering his knowledge for a softer sentence
3) his background as a beastman provides him with a motive for his action, and he has nowhere to go, no family to return to - in desperate times, desperate actions
And while it is true that it is an odd thing to trust an assassin, the reality is that not only is the MC protected by both a powerful fairy and powerful summoned beast but he is essentially pardoning the child and providing him with citizenship which is exactly what he was working for. He will undoubtedly be grateful as he was saved even though he didn't deserve it. Sure, the MC was lenient, but it's for a child that doesn't appear to be evil. That's reasonable enough.
I'm still concerned about the other missions this child has undertaken and what would have convinced the mastermind to trust him with murder but I'm okay with overlooking it.