To foe of His – I'm deadly foe – None stir the second time – On whom I lay a Yellow Eye – Or an emphatic Thumb – (13-16)
Endowed with the power and willingness to kill, the speaker here describes her ability to strike down – literally and metaphorically – any foe of her "Master." The reference to a "Yellow Eye" could refer to giving someone the "evil eye," which is generally a sign of aggression or intimidation. It could also refer to the muzzle flash of a gun, if you want to stretch the interpretation a bit. The last line is similar. Thumbing your nose at someone was once a sign of aggression, and giving someone an "emphatic Thumb" is similar. Just like the "Yellow Eye," it could also refer to the gun metaphor. (The speaker could be calling up the image of the thumb used to cock a gun.)
"For I have but the power to kill, Without – the power to die – " (23-24)
These lines seem complicated, so let’s break them down. The speaker has the power to kill. With her power she is immortalized. Whatever it is that gives her the power to kill, she believes she has it – a belief, which would make anyone feel invincible. If the power to kill make her immortal, then losing the power to kill would rob her of that immortality. This is like saying, without her power, she is brought back to the mortal level and can die. Of course, we all have the power to die, but a passionate emotion like anger can make us believe that nothing can stop us.