The bound morpheme non is the negator for life-threatening here, so 'life-threatening' is more coherent.This does not come across with nonlife-threatening, which would seem to imply a threat to non-life.What is correct in english, non-existing or nonexisting?Searching sources on google doesn't help much as both variants are widely present there.Onelook dictionary search doesn't show much aboutExcept non is not an english word, it is a prefix of latin origin.Which is why american style manuals will always ask you to merge it with the subsequent word, without a hyphen.
Non vs None: Which is the Correct Spelling?
Non vs None: Which is the Correct Spelling?

Non vs None: Which is the Correct Spelling? of Non Pareil Council Bluffs Obituaries: Justice Denied? The Unsolved Cases. .

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British rules differ, and …To record and summarize the discussion in the comments, while the oed mostly uses the hyphen, many other dictionaries don't, and the ngrams show higher non-hyphenated usage than hyphenated.