If the soul is the body, why does Jesus make a distinction between the body and the soul in Mt 10:28?
Jehovah's Witnesses do not believe that the soul is merely the body. The soul encompasses the whole of the creature, mind, body, their life as a person. Here is a Watchtower article that explains the various ways the word "soul" is used in the Bible.
As for Mathew 10:28, which reads: "And do not become fearful of those who kill the body but cannot kill the soul; but rather be in fear of him that can destroy both soul and body in Gehenna", Jesus here is using the word "soul" to represent the person's future life prospects.
Jesus was saying that men may indeed kill us but they cannot determine if we will live again. God, on the other hand, can also kill, but he may also take away any prospect of a resurrection of the dead. Obviously, too, the verse is not saying the soul is immortal, as deluded religionists have been induced to believe. Just the opposite, in fact. Jesus indicated that God could destroy the soul. Basically, Jesus is contrasting two sorts of death. Temporary death and permanent death. Gehenna represents permanent death. Permanent death comes about if God judges a person to be unworthy of a resurrection.