It may look like a pretty flower, but poison hemlock (or conium maculatum) is at heart a deadly weapon. Once ingested, it will paralyze various systems in the victim's body, with paralysis of the respiratory system as the usual cause of death. Worst of all, the mind is active until the last breath--so leave the name-calling until you're sure your subject is unresponsive.
This metalloid element is used in pesticides, but it can do a lot more than just kill insects. Ingesting small amounts over time raises the probability of cancer. For more immediate results, we suggest a high dose which causes stomach cramps, diarrhea, confusion, convulsions, vomiting, and death. Be warned, however: evidence of chronic arsenic ingestion lasts for months or even years later in the victim's hair and fingernails. Dispose carefully.
Macbeth used nightshade to poison an entire army of Danes before becoming King of Scotland, so imagine what the plant can do to a single person. Not only does ingesting it lead to death, but many peculiar symptoms such as complete loss of voice, frequent bending over, continuous movement of the hands and fingers, and dilated pupils. It's said that the poison may be prevented by swallowing a large glass of warm vinegar or mustard and water. So don't keep those around.
Cyanide poisoning is for those who live the fantasy of a mad scientist. Generally delivered in the form of gaseous hydrogen cyanide, potassium cyanide, or sodium cyanide, the poisonous ion makes the cells of an organism unable to use oxygen. In simpler terms: coma, seizures, apnea, cardiac arrest. Beautiful.
Curare is usually used for medical purposes (but when has that ever stopped anyone?), and causes paralysis and death in the "patient" much like the hemlock and strychnine plant. What's different is that even after the respiratory system is paralyzed, the heart may continue beating for some time. So if you're looking for a rather slow and horrific death, well, this one's you're guy.
Strychnine is an alkaloid that paralyzes the victim's respiratory system. Dr. Thomas Neil Cream used itto kill at least seven women and one man between 1878 and 1892. After serving ten years in jail in America, he returned to London to continue his poisoning act. Some say he might even be the same man as "Jack the Ripper." Sound appealing?
A walk in the park can never be so...deadly. That is when you politely offer a mushroom found down yonder. Symptoms vary from slight gastrointestinal discomfort to death. If worse comes to worse, and you've got yourself a tame shroom--just bring it home for some pizza toppings.
If mercury poisoning can a find lad like Jeremy Piven down, well heck, it can get any one of us down. There's a number of exposures that could lead to this poisoning: amalgam dental fillings, eating fish with mercury in its system, industrial exposures such as in the paint industry, and broken thermometers. Nothing that a little day trip to the dentist, market, paint shop, and hospital can't do.
Gas poisoning played a large role of weaponry in World War I, and though a ban was signed in 1925, I'm sure there's a way you smart cats can find a way around that. Recounts from the war share that the deadliest poison gas is mustard gas. The slightest exposure is enough to cause massive blisters. Anything higher can burn a victim's flesh to the bone. Now that's bad to the bone.