I think you should expand more on your statement.
If someone is swinging a knife, regardless of whether they're intending suicide by cop or not, there comes a time at which point they might need to get shot because nobody owes that perp the chance to see if s/he's really bluffing or not.
Getting too close to officers while swinging a knife just might be the point in time when someone needs to get shot. In many other cases, it's when the perp gets too close to bystanders.
If the situation permits, everyone involved would obviously rather not shoot someone. Trying intermediate force adjuncts from a distance (beanbag, OC, taser, etc) as long as you have lethal cover is pretty standard.