According to the last line of the poem Death be Not Proud, why shouldn't death be "proud?"

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."