The poem takes an assertive stand against mortality. It makes the paradoxical statement that mortality is itself mortal. In other words, death doesn’t exist in the long run. But, the speaker wouldn’t make this argument if he doesn’t fear that maybe death is the end. As readers, we must decide whether the poem’s boldness masks some very deep terror about the "void" on the other side of life. After all, is it really possible to talk oneself out of this fear? And, if not, what’s the point of writing the poem?
The speaker’s attempt to intimidate Death fails, because the only thing that can defeat death is Death itself.