Why,according to the poem,should 'death be not proud'?

Death is personified here and the speaker says he should not be proud. He claims that people really don't die after they meet him. The speaker further insults Death by saying Death is no different than "rest and sleep." THe speaker also accuses Death of keeping company with bad company like war and disease.

"death should not be proud" as described in the poem itself that death is indeed not a death instead its a place where one can rest or sleep moreover he declares that death is the only enemy that should be killed

"DEATH should not be proud because it's just a transition stage of a human towards the eternal life. Though death is usually feared by humans, it is something that also dies after having this 'transition'.