In Megillas Esther 3:2 we are told: וּמָרְדֳּכַי--לֹא יִכְרַע, וְלֹא יִשְׁתַּחֲוֶה. Why is this sentence in future tense? Why does it not it say לא כרע ולא השתחוה, in the past tense?
The Mahara"l of Prauge, in his commentary to Megilas Esther called Ohr Chadash, (after offering the more basic suggestion that this denotes something Mordechai would do on a constant basis), explains that even when Mordechai had an option to use an alternate route, he would make a point of going in front of Haman and not bowing down.
The Ohr Hachayim Hakadosh (Rishon Letziyon) takes this idea one step further. He writes that not only did Mordechai not bow down to Haman, but he would actually make a point in showing him that did not wish to. Thus, if Haman happened to walk past while Mordechai was anyhow leaning over, he would specifically stand upright to demonstrate his unwillingness to bow down to him.
The Sfas Emes explains that in every generation, the Jewish nation has one leader who has the soul of Moshe Rabeynu, and will remain steadfast in his belief no matter what tests the people face. This soul is called a "נשמה כללית" - a general soul - as it includes the entire Jewish people (see Rashi to Bamidbar 21:21).
In the particular generation of the Purim story, the "איש יהודי" (the general soul of Jews) was named Mordechai, however the Megillah makes a point of mentioning that Mordechai of every future generation as well "will never bow down" to Haman. Therefore, as the Megilla continues, "It seemed contemptible to him to lay hands on Mordechai alone, for they had told him Mordechai's nationality." Haman realised that just getting rid of Mordechai would never be enough, as such a soul will always exist, and his only hope was to try destroy all the Jews.
(The Sfas Emes there also brings an alternative explanation that not only did he not bow down to Haman, but the possibility to do such a thing never even occurred to him).
R' Levi Yitzchok of Berdichev (Kedushas Levi, Derush Lepurim) offers another fascinating answer, by prefacing another question:
The Megillah writes that Haman became upset, "for they had told him Mordechai's nationality." Certainly Haman was aware that Mordechai was Jewish even prior to this (see there for several proofs)?
He explains that although the king had decreed that all his servants had to bow down to Haman, he actually excluded Mordechai from the decree. The words "מרדכי לא יכרע ולא ישתחוה" are in fact not the narrative after the decree (that Mordechai did not bow), but actually a continuation of the decree (that he did not have to bow). Mordechai could have gotten away with not bowing down without causing any trouble, however he went around telling people that the reason he was not bowing down was not due to his unique exemption, but simply because he was Jewish. Even had he been included in the command, he would have disobeyed and not bowed down. When word of this reached Haman it made him angry and want to destroy the Jews.
Ohr Chadash - Maharal M'Prag asks this question and answers that Mordechai intentionally made sure to be in the areas where Haman was going to show he was not going to bow down.
לא יכרע, זהו אף שהיה יכול מרדכי ללכת בדרך אחרת שלא יהיה פוגע בו ולא יכעס המן