Hdrogen cyanide, or just cyanide.
I'm found in nature, but you'd never know the difference if I wasn't.
Cyanide is found in nature, but in very small portions such as the core of apples, and fruits of the peach family. Wild almonds are also known to contain traces of cyanide (and considered a peach, or specifically te seed of such). However, cyanide is only toxic in seeds of said fruits when consumed in excessively large quantity. Bitter almonds are known for being cultivated with cyanide, so it must be treated and inspected before shipped to the public.
I'm used in plants, but not as the critical part.
Like I mentioned, cyanide is found in the core of fruits in the rose family: this includes apples, peaches, cherries, nectarine, etc.
I'm safe to consume, but you might think otherwise.
Okay so this might be a stretch, since cyanide isn't safe to consume, but really, really small doasges such as those found in the cores.
In excess I'm lethal, but then so is the light stuff!
Pretty explainatory, cyanide kills at small and large doses.