“Listen to me! Are you listening!? Never forget what you did, for today you are a murderer! Let this moment seep into your mind as days go by! Let it shatter you during your sleep as you toss and turn in the comforts of your bed! Allow the blood that stains your hands to shape you into something else! Remember my child, that you’ve killed this miscreant simply because they may look like us, but they can never be like us!”
My first thought was a modern fantasy with hunters that take down supernatural creatures like werewolves or vampires or what-have-you. Teaching their kid how to hunt the bad ones, but reminding them of the weight of it, because it’s still a life lost.