7:9. Mordecai’s life had become valuable to the king because he had saved the King’s life; that is understandable. But what would make Mordecai’s life valuable to Harbonah, one who’s not even a Jew? Being the King’s eunuch and not Esther’s, he would have had few, if any, contact with Mordecai. Maybe Esther had sent him to Mordecai occasionally just as she sent Hathach (ch 4) and so he knew Mordecai.
But more likely, Esther was loved by all of them because she was obviously not vain like Vashti and she honoured their King. More importantly, she probably treated them well, maybe even talking with them about the goodness of her God. Why else would she call her maids to fast with her if they had not come to know the God to whom she fasted? (4:16). And probably she was always speaking of the kindness of her cousin and how valuable he was to her. So maybe, Harbonah was looking after Esther’s interest as an act of kindness too. I don’t know.
But it pays to be nice to everyone, even those socially beneath you.