Question

cyanide poisoning causes death in seconds because

Answer 1

When cyanide is bound to the hemoglobin, the blood can't pick up oxygen in the lungs and carry it to the body's tissues, and the body's cells die from lack of oxygen. You know that inhalation of hydrogencyanide, or ingestion of cyanide causes death.

Answer 2

Cyanide is a such kind of functional group which can readily take up oxygen from our oxygen carriers (hemoglobin) in the blood to form

$CO_2$ & $NO_2$

$HCN + O_2 ---\ \textgreater \ CO_2 + NO_2 + H_2O$

Thus the oxygen carriers don't carry oxygen further more . As a result our body lacks from oxygen and we suffer from deoxygenation .
But it is a matter of minutes not seconds.