Many types of almonds contain trace amounts of cyanide, activated by chewing, but do not contain any arsenic. Some Levantine almonds are activated simply by the handling processes involved in common harvesting.
Any injury to a wild almond seed can lead to the transmutation of its reserves of glycoside amygdalin into hydrogen cyanide, also known as prussic acid. This is a deadly poison which can kill an adult human if they consume even a fistful of the nuts. The breed of almonds is carefully assessed by every grower.
Domesticated almonds do not produce hydrogen cyanide while being processed. They have been bred clear of their glycoside amygdalin over the course of many breeding generations under human care. Almonds bought in stores or at markets pose no risk to their consumers. Wild almonds should never be picked or consumed without oversight and verification of non-toxicity.
Many wild fruits and nuts contain toxic substances which should not be consumed by human beings. These substances are produced as a defense mechanism to ward off overgrazing and feeding by animals and insects, keeping trees, vines, and bushes healthy and capable of regular growth. Human breeding has eliminated these qualities in many plants now farmed for food.