"Death, be not proud" is a sonnet, divided between the first eight lines (octet) and the remaining six (sestet). In traditional sonnet form, the octet presents a problem that is resolved in the sestet. The transition from the last line of the octet to the first line of the sestet is called the volta, or turn. This is a shift in the argument or the subject matter of the poem. The ninth line of a traditionally-structured sonnet is therefore pivotal; a problem has been presented and now it's time for a resolution.
In "Death, be not proud" the ninth line is as follows:
Thou art slave to fate, chance, kings, and desperate men,
In this line we can see the speaker's contempt for death increase. There's a greater sense of urgency from now on, a much more vigorous, attacking tone. In the first eight lines, death has been presented in negative terms (not proud, not mighty, not dreadful, etc). But now the speaker tells death not what it isn't, but what it is, and that is a "slave to fate, chance, kings, and desperate men."