Take it to the extreme: if you were enslaved, forced to care for critically ill people, would you be acting wrongly to run away if you could? I don't think so. The difference is a matter of degree, not of kind.

More practically, not every nurse's threshold is in the same place; each nurse decides they've had enough at a different point. It's not as though — barring coordinated effort — every nurse is going to quit the same day. The problem will manifest itself as declining staffing, not a sudden depopulation of the hospital, and it will not go unaddressed.