Not that I bid you spare her the pain!
Let death be felt and the proof remain;
- Even though she wants to make sure the poison works, she isn't going to let this other lady die easily.
- She wants her to feel it when she dies, and she wants everyone to be able to see the "proof" of that painful death when she's gone. (Ugh! Pretty intense, isn't it?)
Brand, burn up, bite into its grace—
He is sure to remember her dying face!
- She wants this painful, terrible death to burn and scar and chew on this woman's face, to destroy all her "grace." Not only because she wants to hurt her, but because she wants her former lover to remember his new girlfriend's agonized "dying face."
- Just for the record, we're scared of this lady.