Maybe you've already noticed how much of "The Laboratory" is about death—not just the idea of death, or some poetic thoughts about mortality, but the dirty, violent reality of poison and murder. Our speaker has fantasies about killing not just one person, but a bunch of people. And when she dwells on all the nitty-gritty details, you can tell that she's excited by the idea of deciding when other people live and die. Murder isn't just a way to get what she wants; it's something that she actually enjoys. She's an enthusiast about killing people in the same way some people enjoy collecting stamps, or raising ferrets. And that makes the poem all the more disturbing.
Like a linguistic blender, the poem creates its creepy effect by mixing death and life, contrasting images of gruesome poisoning with the lively, youthful excitement of the speaker.
The poem works like a good horror movie (like Friday the 13th Part I, or Friday the 13th Part II, or Friday the 13th Part III, or…). It gives us the thrill of death and danger, but still allows us to have a good time.