Sentence:  Such a scandal as the prosecution of a brother for forgery—with a verdict of guilty—is a most truly horrible, deplorable, fatal thing. It takes the respectability out of a family perhaps at a <WSD>critical</WSD> moment, when the family is just assuming the robes of respectability: ... it is a black spot which all the soaps ever advertised could never wash off. 
Meaning of the word marked with <WSD> tags:  Pertaining to, or indicating, a crisis or turning point.