Can atrial fibrillation be life threatening?

Indirectly: It can lead to stroke or heart attack of left untreated.

Yes: Atrial fibrillation increase the risk of stroke, which can be disabling or fatal. Strokes from af tend to be worse than non-af strokes. Additionally, if the af causes a fast pulse for a prolonged period of time, this may lead to weakening of the heart and cause heart failure. A very fast pulse, even for a short period, can precipitate a heart attack in someone with underlying blocked arteries.