lethal vs deadly

lethal and deadly both are adjectives.

Word	Noun	Adjective	Verb	Adverb
lethal	No	Yes	No	No
deadly	No	Yes	No	Yes

As adjectives, lethal and deadly are synonyms defined as:

lethal (adjective)	deadly (adjective)
of an instrument of certain death	exceedingly harmful
	extremely poisonous or injurious; producing venom
	involving loss of divine grace or spiritual death
	(of a disease) having a rapid course and violent effect
	causing or capable of causing death
	of an instrument of certain death

lethal (adverb)	deadly (adverb)
	(used as intensives) excessively
	as if dead