I've read some comments implying that Moon would be kidnapped or killed, but if North Korea wants stability on the peninsula, they obviously aren't going to have the South Korean leader killed on their turf. That would basically be suicide for their government.
Yeah, if any sort of thing like that were to happen, I'd bet on the US doing it. North Korea knows that any sort of violence to that level would mean its immediate annihilation. They aren't stupid. If you look at the way they've been acting for the past decade or so, you'd see a fairly rational and predictable actor.