The poem makes a mockery of the belief that Death is all powerful and consuming by firstly addressing it as human (personification) and then criticizing its arrogance. The speaker states that Death should not be boastful about the fact that it has been called "mighty and dreadful" by some, for it is not. The speaker contends that Death's belief that it has ruined life is not true and that it cannot overcome him.
The speaker belittles Death by equating the images of "rest and sleep" to the likeness presented by Death. Such rest and sleep bring comfort and peace much as Death does when it removes the greatest and most respected individuals from their physical state. All Death does, then, is to deliver their souls to a place of eternal rest. The speaker mentions that Death has to do much more to claim its status as the powerful and terrifying force it assumes itself to be.
Furthermore, Death's claim to glory has no foundation, for it is ruled by chance and is a victim of fate. It is just as much controlled by the command of kings and is subject to the desires and whims of "desperate men." In essence, then, Death is not as much in control as it claims to be. In addition, Death cannot be boastful for it is associated with perfidy in the form of sickness, poison, and war—surely not companions one can be happy with. To accentuate the fact that Death has no real authority, the speaker states that drugs and spells can be used to bring about sleep even better than Death does, so it has no reason to brag.
In the end, Death has no dominion, for its power is temporary. The speaker makes it clear that once the soul awakens from its control (a temporary sleep) it will live forever and Death will cease.