I was wondering how cyanide kills so fast.
I did some google search and found that it kills by depriving organisms of ATP and oxygen.
CN ions bind to cytochrome enzyme in mitochondria and turns off production of ATP. Cells die without energy [which include heart and brain cells]. It also deprives cells of oxygen by preferentially binding to Heamoglobin over oxygen, so it ends up transporting CN
It explains quite a bit of details:
http://publications.drdo.gov.in/gsdl/collect/defences/index/assoc/HASH016b/2155e82b.dir/doc.pdf
