life-threatening vs severe

life-threatening and severe both are adjectives.

Word	Noun	Adjective	Verb	Adverb
life-threatening	No	Yes	No	No
severe	No	Yes	No	No

As adjectives, life-threatening and severe are synonyms defined as:

life-threatening and severe: causing fear or anxiety by threatening great harm

Other synonyms of life-threatening include dangerous, grave, grievous, serious.

life-threatening (adjective)	severe (adjective)
causing fear or anxiety by threatening great harm	causing fear or anxiety by threatening great harm
	intensely or extremely bad or unpleasant in degree or quality
	severely simple
	very strong or vigorous
	very bad in degree or extent
	unsparing and uncompromising in discipline or judgment