Death and evil are equated in this verse. The Psalmist is consistent with other writers in the New and Old Testament in grouping death and evil together. The author of life is God the Father. The author of death is Satan. Satan came into the world and brought death to the world. Satan brought pain and suffering into the world. All of walk in the shadow of death for we all know we are going to physically die at some point. What we must also know is our death in the physical body need not be death in our heart, soul, or spirit. We need not have any fear of death for Christ brought an end to the death of our spirit when Christ died for us on the cross. Christ is indeed with us if we chose to be with Him. Christ is the protector of our soul or spirit.
The shepherd who keeps watch over his flock keeps a rod or club with him, along with a staff. A staff is a long pole with a hook on one end of the pole. The shepherd uses the staff to pull a sheep out of crevices in the rocks, out of bushes, briars, and dangerous places. Sheep have no real defenses against predators and are nearly helpless for sheep have limited physical abilities. We call Christ our shepherd for Christ protects us in many ways like the shepherd protects the sheep.
Human fear is a natural occurrence. We tend to experience more and more fear the more pain we suffer.