Examining the simple things that were never really questioned

Personally, I feel like as a child when I read the Purim story, I kind of just accepted that Mordechai did not bow down to Haman. Anyone else with me? I always thought, " Well, everyone else bowed down to Haman but Mordechai just decided he did not want to so he didn't. When we were learning the Megillah in class, I started to think about it. Seriously, why couldn't Mordechai just make one simple movement because in all fairness, that was the decree? After I realized that Mordechai did not bow, I was kind of bothered. Was there a logical reason? Well to me, so far I have not seen any logical reasons.
     We discussed in class how we will eventually see the reason for Mordechai rejecting the decree.
a. Is it possible that there was tension before with Haman and Mordechai?
b. Is it possible that Mordechai felt it was against his religion?
c. Is it possible that Mordechai was just extremely stubborn and decided not to bow down just to get his way?
d. Is it none of the above?
There are so many possiblities for the reason to the simple question asking why Mordechai would not bow down. I feel like knowing the answer to this is a key part of understanding the Megillah.
I then have a second question. We learned that Haman did not even see Mordechai disrespecting him, his guards saw it. Well, if there were so many people in the empire did we ever think about why the gaurds were specifically just watching Mordechai?
I feel like this is a quiz, but I am actually extremely curious. A lot of the times we kind of just except the small things, but I feel like us being clear on these little things is a key factor of us understanding the whole Megillah.
And now I want to know. What are your thoughts?