"And do not fear those who kill the body, but cannot kill the soul; but rather fear Him who is able to destroy both soul and body in Gehenna (hell).” (Matthew 10:28)
Your translation of the Bible probably renders the word "Gehenna" as “hell”. That is unfortunate and an example of how the Bible was translated into English according to the bias of the translators. This bias promotes the vain "directions of men" rather than the Word of God.
"Yet in vain are they revering Me, Teaching for teachings the directions of men." (Matthew 15:9)
"Gehenna" refers to the garbage dump outside of Jerusalem where trash and dead bodies of animals and criminals were burned. Among that garbage, the worms did not die and the fire was not quenched until it had accomplished the job of destroying the body.
"And if your hand should ever be snaring you, strike it off. It is ideal for you to be entering into life maimed, rather than, having two hands, to come away into Gehenna (hell), into the unextinguished fire where their worm is not deceasing and the fire is not going out." (Mark 9:43-44)
Following is what I think Jesus was saying about "fear" and "Gehenna"?
Jesus warned the Jewish people that embracing Him as the Messiah could result in death, with the dead body being cast into "Gehenna" as a criminal - the garbage dump outside of Jerusalem where many bodies of criminals were placed after execution. Yet, Jesus told them they need not fear men who could do this because that is all they could do. Jesus said only God had the power to destroy both body and soul in the fire of "Gehenna" - therefore there should "fear" Him only.
"And the souls of those executed because of the testimony of Jesus and because of the word of God, and those who do not worship the wild beast or its image, and did not get the emblem on their forehead and on their hand - they also live and reign with Christ a thousand years. The rest of the dead do not live until the thousand years should be finished." (Revelation 20:4-5)