The Ginzei HaMelech uses this verse to show how humble Mordechai was, in that this giant of his generation still lowered himself to serve Esther like a servant. He writes further that this act was in direct defiance to Achashverosh’s earlier decree that men should be in charge of women (Esther 1:22).
Perhaps this use of language is what Tanna D’vei Eliyahu refers to when it writes that Esther spoke with Mordechai in a disrespectful manner.
17. And Mordechai passed and did like all that Esther commanded on him.
According to the Talmud (Megillah 15a), after hearing Esther’s response, Mordechai passed over either a river or the passed over (read: transgressed) the obligation to eat on the first night of Passover, since that night fell within the three days in which Esther asked the people to fast.
The Me’am Loez writes that the verse is praising Mordechai for “crossed the river,” which implies that he preferred to follow the command himself – without the use of messengers. He, himself, crossed the river to gather the Jews together in prayer, fasting, and repentance.
In explaining why Rashi, who usually gives a simpler explanation when available, decided to write the explanation that had to do with transgressing Pesach, the Torah Temimah gives two reasons: one is that the Torah always names an object being crossed when vaya’avor is used in relation to a physical object, and secondly, in actual fact, the fasting did occur through the first night of the Pesach seder.
R’ Dovid Feinstein points out that someone can fast on Shabbos to annul a bad dream if that dream is dreamed on that day (Talmud, Shabbos 11a). If so, how much more-so can one fast on Yom Tov to annul a decree, so Mordechai was not transgressing at all.
However, according to the opinion that he was, R’ Simcha Bunim of Peshis’cha explains that Mordechai felt his prayers would not be powerful enough to be listened to in the ordinary manner. Transgressing Pesach would get the Accuser, the Satan, involved. Once he gets involved, there would be a Heavenly tribunal. Once there is a trial, Heaven would recognize Mordechai’s good intent, and then would assist Mordechai in defending the Jews.
In the first chapter of Tanna D’vei Eliyahu, it says that H-Shem can ignore insults. There, it writes that Esther’s arguing was spoken in an unfit manner, and yet Mordechai let it “pass” from his mind.
M’nos HaLevi points out that crossing the river was as easy for Mordechai as jumping over a puddle. It was a small act, but the Torah records it for our benefit, so teach us the lesson of the power even in what may appear as minor, easy mitzvos (see Mishnah, Avos 2:1).