Matthew 10:28 says, "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 hell."
The word "soul" in this context implies "the capacity to live." That is, another human being may be able to take away our life, but we still have the capacity to live again. God can always bring us back to life in the resurrection. So, though we naturally fear people who can cause our physical death, Jesus was giving us the true, eternal perspective. We should not fear man, but God.
Why? God has the authority to deny bringing us back to life—destroying the capacity to live ever again. "Hell" in this verse refers to the gehenna garbage heap fire burning in the valley outside of Jerusalem. It's symbolic of what the Bible calls "the lake of fire." If we die "the second death" in the lake of fire, there remains no hope of a resurrection (Revelation 21:8).
If we fear—reverence and obey—God, we don't need to fear men.
For more insight, please read our booklet Who Is God?