What a drop! She's not little, no minion like me—
- In this stanza, she worries about there not being enough poison to do the job. She calls the potion that this chemist has mixed up a "drop," and reminds him that the intended victim isn't nearly as dainty as she is (there's that catty jealousy again).
- Basically, she's letting him know she thinks it'll take a lot of poison to kill her boyfriend's fat new girlfriend. Double-ouch.
That's why she ensnared him: this never will free
The soul from those masculine eyes,—say, "no!"
- Not only is the new girl heavy, but our speaker thinks she looks like a man too, with "masculine eyes." It sounds odd to us, but apparently that's the only way she could trap ("ensnare") her guy.
- Again, our speaker wants to up the dosage on the poison, to make sure this woman's soul and body are separated for good.
To that pulse's magnificent come-and-go.
- Even though the subject matter is disturbing, Browning makes it all sound kind of pretty. All our speaker wants to do is stop this other lady's heart, but she phrases it in a beautiful, dramatic kind of sing-song. She wants to "say, no!" to the "magnificent come and go" of this lady's pulse.
- Just like with the blue and gold poisons (14-15), this makes all this dirty work sound appealing.