An additional theme to add to the other response is the power to face death with a steadfast courage. When most consider death, it is with a trepidation of the unknown. Yet this speaker shows no such fear:
...some have called thee
Mighty and dreadful, for thou art not so
Death is the end of all that we know in this world. So for the speaker to be able to look at this uncertainty and declare that Death has no real power and is neither "mighty" nor "dreadful" shows courageous tenacity. The speaker goes on to say that death "nor yet canst...kill me," his fearlessness showing a defiance of Death's claim to power.
The speaker denounces Death as a "slave," rendering its power useless under kings and even "desperate men." The speaker uses literary apostrophe to speak directly to Death, and this in itself shows a courageous spirit when compared to other voice options, such as writing about death in third person. This very direct and confrontational voice is somewhat antagonistic, ending in one final promise: "Death, thou shalt die."
Together, the voice, tone, and word choice show that the speaker is courageously certain of eventual victory over Death, leaving no room for any other possibility. There is no "unknown" to be questioned, as the speaker considers Death a mere transition from this life to one of eternal life. Thus, Death can never claim the speaker.