It says in the Megilla "ויאמרו עבדי המלך אשר בשער המלך למרדכי מדוע אתה עובר את מצות המלך" - "The servants of the King (who were in the King's gates) asked Mordechai 'Why do you go against the command of the king'"?
It's known (Esther Rabbah 3:10) that "the King" in the Megilla refers to Hashem. Why did Mordechai do an Aveira?
I once heard an explanation - will have to see if I can find the source - that these other servants (Jews, presumably) held with the opinion later expressed by Rambam (Hil. Yesodei Hatorah 5:1,4) that one who risks his life to keep mitzvos when not required to do so (i.e., when it's not one of the "big three," the non-Jew is doing it for his own benefit, it's not in public, and not at a time of religious persecution) is "liable for his own death."
In this case, then, the bowing wasn't being commanded as an act of idolatrous homage (or, even if it was, it might have been a case of עובד מיראה - see Tosafos to Sanhedrin 61b, ד"ה רבא). They thus protested that Mordechai is transgressing the halachah in endangering his life for this purpose.
(His reply, then, was אשר הוא יהודי - he is a prominent Jew and as such has to hold himself to a higher standard to make a public kiddush Hashem - see the second answer of Tosafos ibid.)
My first guess would be: "the servants of the King", i.e. the other rabbinic leaders, asked Mordechai why he violated the will of the King by abandoning his rabbinic role and getting involved with politics.
Even if an unqualified "king" in the Megillah refers to Hashem, the instance you are citing is a quote of Achashverosh's servants. They were surely referring to Achashverosh.
ואמרו חכמינו ז"ל במדרש כל מקום שנאמר המלך סתם הוא הקדוש ברוך הוא וכל מקום שנאמר אחשורוש באחשורוש ממש מדבר. No idea what midrash he's referring to, though. Maybe he's just generalizing the above references from Esther Rabba.