According to the last line of the poem Death be Not Proud, why shouldn't death be "proud?"
Death is regularly personified in poetry and in John Donne's Death Be Not Proud, apostrophe (a form of personification) is used as the poem addresses Death itself. Death fascinates humankind and Donne wants to break down the all-powerful image of death as a victor and as having the last say. He wants readers to understand that death is only the beginning of another phase and needs to be viewed that way so as to prevent death from consuming all rational thought. Therefore, he addresses death so that he can launch his attack on something tangible, making his assault more effective.
Death is a confusing phenomenon and is paradoxical as the people left behind after the death of a loved one feel loss and grief but find solace in the vision of what lies beyond death. Any comfort is surface deep because death is "mighty and dreadful" and Donne wants to break down that perception and remind people that death is like sleep and his confidence is intended to convince the reader that he or she will definitely "wake eternally."
Donne is therefore leading up to his ultimate triumph. He belittles death suggesting that it has no control and is at the mercy of (or "slave" to) other forces such as "fate, chance, kings, and desperate men" and death is only part of the picture because it can be found in places where "poison, war, and sickness dwell." He delivers his final insult by asking death whether it still thinks it should "swellest" when it has become clear that the only loser is death itself and "Death thou shalt die" meaning that even death cannot actually die because Donne has already reminded readers that death is only the path to eternal life. There is nothing to be proud of as death does not achieve its aim.
The paradox of Death dying serves as the conclusion to the argument that Death should not be proud because other forces such as poison, war, and sickness--even drugs--take the lives of men fatefully and "better than thy stroke," and Death is their slave. And, above all, man survives Death's arrival by attaining eternal life.
In a series of paradoxes, John Donne, himself an Anglican minister, attacks the prevailing idea of Death as invincible. The underlying conceit of "Holy Sonnet 10" is the likening of death to a proud but ultimately ineffectual tyrant who thinks that he overthrows man, but only gives man "One short sleep." With the belief in the afterlife that comes after "one short sleep past," man's soul lives on, waking to eternal life; therefore, Death is defeated: "Death, thou shalt die."