Is the Paratrooper’s body a symbol of good, evil, or something else?
I am assuming you are asking about the pilot's body that floats down from the plane that eventually lands near the top of the mountain and frightens the boys severely.
In some ways it is a symbol of evil, connected to the war happening around the world, a symbol of the death and destruction that is brought to the entire world because of that conflict.
For the boys it may symbolize the beast, something they cannot quite pin down that eventually is shown to be the evil inside of them, but this thing that they can't quite see that moves and appears to have a life of its own and perhaps is bringing all this difficulty and confusion to them serves as a symbol of evil.
But as it is blown out to sea by the wind after Simon's death, it may have become a symbol for the false hope that the evil is outside of them and can be banished by killing when in fact it has become apparent that the evil is inside of them and has only been strengthened by their killing of Simon.