Death is the unknown, which always threatens the known, the patterns you have created.
The paradox is, that you can´t be afraid of the unknown, because you don´t know what the unknown is, and therefore there is nothing to be afraid of.
Death is a word, and it is the word, the image, which creates anxiety. The word is a manifestation of your self-image and world-image.
For as long as the image exists - the image from where the thought origins - the thought must constantly create anxiety.
Then you rationalize your mortal dread and build a defence against the inevitable, or you invent countless faith-conceptions, which can protect you against the anxiety of death.