Esther in danger of her life followed the grace of virtue; nay, even a heathen king did so, when death was threatened to a man most friendly to him. For friendship must ever be combined with virtue, as the examples of Jonathan and Ahimelech show.
123. Why did Queen Esther expose herself to death and not fear the wrath of a fierce king? Was it not to save her people from death, an act both seemly and virtuous? The king of Persia himself also, though fierce and proud, yet thought it seemly to show honour to the man who had given information about a plot which had been laid against himself, to save a free people from slavery, to snatch them from death, and not to spare him who had pressed on such unseemly plans. So finally he handed over to the gallows the man that stood second to himself, and whom he counted chief among all his friends, because he considered that he had dishonoured him by his false counsels.
124. For that commendable friendship which maintains virtue is to be preferred most certainly to wealth, or honours, or power. It is not wont to be preferred to virtue indeed, but to follow after it. So it was with Jonathan, who for his affection's sake avoided not his father's displeasure nor the danger to his own safety. So, too, it was with Ahimelech, who, to preserve the duties of hospitality, thought he must endure death rather than betray his friend when fleeing.
Source: On the Duties of the Clergy (New Advent)