If "Death be not proud", then - what is it?
Does not "pride cometh before a fall"? And don't you fall when you are dead? Down. Unto, and then under, the ground.
This seems to be one of those Eternal conflicts which can not be solved until you are, well, dead. And then if you do solve it, you apparently can not take any pride in the fact. According to Donne. But then, he's done.