The poem begins by naming the speaker’s greatest fear, followed immediately by a command, as if Death is one of his servants. This is meant to strike us as gutsy and inspiring. But, keep in mind that the speaker only has his words and thoughts to fight with are.
And soonest our best men with thee do go, Rest of their bones, and soul’s delivery. (lines 7-8)
In these lines, "best" might as well mean "bravest." The people who are most likely to "go with" Death soonest are the people who risk their lives, which is the definition of physical courage. But, there’s a twist: the poem seems to say that these brave people, including soldiers and martyrs, know something that the rest of us don’t; namely, that death will bring rest and freedom. Isn’t the point of courage, though, that it involves a risk for the person who does a courageous deed? Where is the risk here?
Thou’art slave to fate, chance, kings, and desperate men, And dost with poison, war, and sickness dwell,
"Guts" or "nerve" seems more accurate than "courage" in describing what goes on in this line. The speaker doubles down on his bet by insulting Death more directly.
And poppy’or charms can make us sleep as well And better than thy stroke; why swellst thou then? (line 11-12)
Is it courage or delusion to compare Death to a drug that many people in Donne’s day consider pleasant and relaxing ("poppy" is a reference to opium)? The speaker seems to try to make death into an experience that requires less courage than is normally thought. After all, anyone can sit around and smoke an opium pipe. At least, in the 17th century, before they know how bad drugs are for you.