Depends on how you quit. If a nurse or doctor gives appropriate notice before quitting or striking, so that innocent patients can be rerouted first, then there's no problem. But if they're walking out of an ER room and abandoning a patient on the table with their rib cage splayed open, that's basically murder.

In this case, the nurse who "called 911" was faced with a situation where the waiting room was packed with people who needed emergency care. For her to walk away from her job in that moment would be like leaving somebody on the ER table. At the very least, a nurse in that situation should persist through their shift and quit afterwards, not quit on the spot.