"To foe of His – I'm deadly foe – None stir the second time –" (17-18)
This section refers to the speaker’s power to kill. She defends her "Master," implicitly killing any of his enemies. On a literal level, it actually means shooting and killing someone, since "none stir the second time" – because they’re dead. On a metaphorical level, "none stir the second" time could mean that they don’t challenge her "Master" a second time. They won’t necessarily have to die, but they may be intimidated by the speaker.
"Though I than He – may longer live He longer must – than I – For I have but the power to kill, Without – the power to die – " (21-24)
While the threads of death and violence are woven throughout the poem, this last stanza is the most deeply involved in the theme of the speaker’s mortality. It implies that someone or something gives the power to kill to the speaker. Though she may live longer than him, he must live longer, because without the power to kill, she has the power to die. This is a very tricky stanza to understand. One reading would be that without the power to kill, she is left with the power to die. This would mean she has lost immortality. It could also mean that without "Him," she loses the power to kill and gains the power to die, therefore he "must" live longer than she lives. This could also relate to the mortality of the poet. Often the poem itself is seen to be part of the poet, an extension of his or her life through its continued existence. Often we say of a poet: "His life is immortalized in his work." Thus, the "death" of the poem could mean the death of the poet.