Nicotine is an Alkaloid poison. It gives you hallucinations as it kills you. As a way to go it is better than being cut into chunks.
I can remember reading the tobacco thing on the jolly roger's cookbook years ago.
Maybe a couple of kilos to go in the preps.
I regular supply of antifreeze would do the job if left in a pop bottle.
Its very sweet to taste so you would never know until you were close to death with liver and kidney failure.
The natural way would be deadly night shade (flowers and sap)