The individual who raised them was a child who everyone in the village raised (irony?) because he was abandoned.
Despite sounding similar to a Twilight Zone episode, this is still a good idea. I would have the kid in a state of denial. He knows what he has done on an instinctive level, but hasn't ever acknowledged it out loud.
This idea, on the whole, has appeared in a number of films and books over the years. In most cases, the people are physically-manifested ghosts - that is, they are physical, and can be touched and interacted-with, but there is something "off" about them. They are too quiet, sometimes space out, and sometimes manifest strange wounds that either reappear and disappear, or never disappear once they are discovered by the protagonists.
You don't need a specific creature to do this, nor rules beyond giving them a low NPC level. If the PCs hurt or kill them, the body just disappears when they aren't looking, then reappears later. See above about the wounds. You don't need a DC for Will saves, either. Describing their behavior in a moody, freaky way should be good enough (horror or macabre games are not about making the characters uncomfortable, they are about making the players uncomfortable).
In some stories, the dead people start disappearing in the order they died, acting as a clue to the protagonists as to what is really going on (provided they find the right newspaper clippings, or diary to match the clue to).
Most of the time, they need the protagonists to do something to help them find peace, but in their efforts to convince them to help, end up terrorizing them, instead. Shades of Sixth Sense there, but still a common trope.