Deadly, deathly are frequently confused although in precise use they are not synonyms.
Deadly applies to an agent which is bound or extremely likely to cause death (see DEADLY 1); in one of its extended senses, it applies to something which is so implacable or virulent or so relentless that it can result only in death, destruction, or ruin.
Deadly may imply no more than an extreme of something or it may suggest a disgusting extreme of some depressing or spirit-destroying quality.
Deathly applies only to that which suggests the appearance or the presence of death.