severe vs life-threatening

severe and life-threatening both are adjectives.

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

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

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

Other synonyms of severe include dangerous, grave, grievous, serious.

severe (adjective)	life-threatening (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