gesundheit. If you want to kill someone slowly, I hear white arsenic is a good way to do it. Initially, it causes gastrointestinal problems, and eventually causes liver and kidney degradation; contact with eyes will be very bad. https://en.wikipedia.org/wiki/Arsenic_trioxide
I'm kind of disturbed how deep into this I've gotten. I've been flipping through my book of poisons and it's incredibly informative. And ethylene glycol, while toxic, is still not as toxic as some of the plants given. Foxglove is brutal. I think plants are more easily accessible than some of the chemicals, although with the ethylene glycol it's probably more readily available. That said, there are remedies for antifreeze poisoning, and a lot of people survive this if caught in time. I'm not sure where I was going with that, but this topic has been endlessly fascinating.
Unless your MC has a well-fitted lab and the chemistry knowledge to make it, my comment won't apply, but . . . nerve agents (acetylcholinesterase inhibitors). Most painful and horrifying-to-watch way to kill any character. To remember what it does to people, remember the acronym SLUDGEM: salivation, lacrimation (tears), urination, defecation, GI upset, emesis (vomiting), miosis (pinpoint pupils). Doesn't sound too bad, but imagine pissing and shitting and vomiting yourself to death.
Venom from a snake or spider might work well, Very liquid and many are extremely fast-acting. Could 'milk' either one against the edge of a glass the way the snake handlers do.
I had suggested pufferfish before realizing i was reading page 2 like it was page 1. Someone already suggested it.
What year does your story happen in? Some of the suggestions here, how widely known are these options by your characters? I personally in 2020 do not know which snakes are venomous nor which spiders are poisonous, at least not without Google's assistance. I also suspect one needs specialized knowledge to "milk" a snake or a spider. It's true that you can fictionalize some elements of your poison but it's a bit implausible that your character in location X who has never had higher education or travelled outside of his village is going to have the knowledge to manufacture and administer an exotic poison. This is something that always bothers me in some stories.....yah, right, he lives in rural Iowa and he just happens to have access to pufferfish toxin....yah....
I suppose an easy way to go about it would be to grind up about 5 or 6 Xanex to slip in the drink. Of course, if you victim is a big guy, it might take a double. But about 2 grams and two fingers of Jim Beam will take out your average sized person.
I know brown recluses are the dangerous spiders around these parts, and most of us know how to identify them (because many of us have been bitten or know people who have). I also know what a black widow looks like, though they don't live near me. For snakes, I think we all know to steer clear of anything that rattles at you. Beyond that I 'think' water moccasins and cottonmouths are poisonous (not sure), and from Kill Bill I understand the Black Momba is pretty badass and apparently wears a yellow jumpsuit with black stripes down the sides. And if you've ever sat and watched Animal Planet you could definitely have seen how snakes and spiders are milked.
In one of my stories the islander hunted by WWII Japanese, poisons his pursuers with psychedelic mushroom powder, frog toxin, and scorpion venom. And a candle. They died in freakish, nightmarish agony, thinking they were on fire! After a few days patrols would not go near the area. He was really pissed off that they killed the entire population of the island.
This. Makes me think of Lost World where Eddie Carr has this short conversation: [about the poison on the darts in their guns] Eddie Carr : The most powerful neurotoxin in the world. It works faster than the nerve conduction velocity, which means the animal's down before it actually feels the - P! - prick of the dart. Dr. Ian Malcolm : Is there an antidote? Eddie Carr : What, like if you shot yourself in the foot? Don't do that, you would be dead before you even knew you had an accident. SB
https://www.indiewire.com/2018/08/the-sinner-season-2-episode-1-part-1-recap-spoilers-derek-simonds-1201990204/
If it's set in a certain time period, something like Aqua Tofana, a branded, commercially available poison from renaissance Sicily, would be an interesting approach.
High velocity aspirin to the forehead maybe? Toss a blue ringed octopus that is really irritated at them. Unless you can figure out a way to get the neurotoxin from the cute little guy, and put it in a dart or something pointy.
Individual style is up to the person with the piano wire. I do remember a case I studied where,back in 1953, a woman strangled her husband's mistress with his jock strap. I'd call that worth some style points.
https://www.tellwut.com/surveys/trivia/131280-a-piece-of-the-rock.html There's an interesting piece about ancient Chinese people dipping their arrows into the chemical soup from the rock to coat it with poison. The orpiment part. This survey is the first I've ever heard of it.
Cyanide is quick. I once watched some footage from a courtroom where a guy was up for a large insurance fraud (I think he burnt his mansion down). As an overly harsh sentence is passed you see him raise a hand to his mouth. About three minutes later he slumps, snorting like a pig, into his lawyer sat next to him (snorting not because he was asleep but because his respiratory system was shutting down). He was rushed to hospital but they failed to revive him. It was rather haunting to watch.
Iocane powder. It is odorless, tasteless, dissolves instantly in liquid and it is among the more deadly poisons known to man. Luckily for me, I spent the last few years building up an immunity to iocane powder.