I don't see why anyone who has rationally considered death would fear it- at least not for his own sake. Fear is an emotional state derived from a feeling that some object or action is a threat to oneself. Death, however, poses you no threat, because it is impossible for you to experience your own death. The process of dying may be something to fear. For example, if you get stabbed, it will surely hurt a lot. I think it's reasonable to fear that pain. However, death cannot be painful, nor can it be harmful to the person who dies. Pain and harm can only come to someone who is alive.
I think it's sometimes reasonable to be averse to death for other reasons. For example, if your death would lead to an increase in suffering, then it is reasonable to want to avoid dying. But, if your concern is solely with the fate of someone else, I'm not sure whether it can really be considered "fear".
