Could I keep them one half minute fixed, she would fall, Shrivelled; she fell not; yet this does it all! (35-36)
For a moment, our speaker fantasized that she could kill this other woman just by looking at her, by channeling all of her hatred into her gaze. (This lady obviously has a really vivid imagination.) It didn't work (big shock), so now she's stepped up her game. She's turning to poison because she thinks that will finally accomplish her goal for certain ("this does it all").
Let death be felt and the proof remain; (38)
This line ties into the same feelings she was expressing in lines 27-28. The speaker doesn't just want revenge (although she definitely wants that, and in the worst way). She also wants her revenge to hurt ("let death be felt"). Beyond that, she wants everybody to see the evidence ("proof") of her terrible revenge. She wants them to know that her boyfriend and his new lady messed with the wrong person. This plot, when it's done, will be the proof of how deadly, cunning, and powerful she really is. Mwahahahaha.