In the ending the boy approaches his sister, who is picking flowers by a rope ladder to some treehouse. The title screen is that same area, only the rope ladder is broken and flies are hovering over what appears to be dead bodies. Theirs?
Is she picking flowers, or crying? Why is she crying?
If Limbo, the outermost edge of Hell, is this threatening and scary... then what's the rest of hell like?
Remember the giant boulder that comes rolling toward you early in the game? That wasn't a boulder. It was another limbless spider body.