Deadly, mortal, fatal, lethal mean causing or causative of death.
Deadly may imply an extremely high degree of probability rather than a certainty of death; the term therefore applies to something with the capacity of or a marked potentiality for causing death; a deadly disease is one usually ending fatally; a deadly weapon is one capable of inflicting death; the seven deadly sins in theology are those sins which must be avoided because they are the source of other sins and are destructive of spiritual life and progress.
Mortal implies that death has occurred or is certain to occur; the term therefore is applicable only to that which actually has caused or is about to cause death; a mortal disease is one that ends fatally; a mortal wound is one that caused or will inevitably cause death; a mortal sin (in contrast with a deadly sin) is a grievous sin deliberately committed and actually inflicting spiritual death. Deadly applies to the instrument that deals the wound or blow that proves mortal.
Fatal stresses inevitability and applies to a potential or actual result of death, destruction, or disaster. The term is often used in place of mortal as applied to wounds, blows, or illnesses especially when some time has intervened between the wounding or sickening and the dying.
Fatal rather than mortal is used in predictions.
Lethal applies only to something which by its very nature is bound to cause death or which exists for the purpose of destroying life.
All of these terms except lethal may be used in a lighter sense not implying physical or spiritual death, but something dreaded or greatly feared; a deadly shaft of irony causes complete discomfiture; mortal terror always suggests extreme terror, but only occasionally the terror of losing one's life; a fatal error or a fatal slip may imply the destruction of one's plans or hopes rather than of one's life.