why swell’st thou then?
In medieval Christian theology, pride is the most dangerous and seductive sin. It is the source of almost all other sins. So, to say that Death "swells" with pride is a serious accusation. Whether or not Death actually cares is another question.
One short sleep past, we wake eternally, And death shall be no more; Death thou shalt die (lines 13-14)
Now, finally, the poem becomes really religious. The setting shifts from the present to the future, as the speaker describes what will happen after death. This is funny, because so much of the poem is preoccupied with imagining what death itself is like. Here, he finally puts death behind him – figuratively, at least. This is like the end of an action movie, when the hero finally offs the villain – but not before uttering a snappy quip.