The Vilna Gaon points out that, had Haman seen Mordechai not bowing to him, the servants would have no need to report this information to him. However, filled with self -love and pride, with his chin in the air, Haman never even bothered to see this phenomenon for himself!
Megillas Sefer says that the king’s servants were acting out of a sense of curiosity. In the Book of Daniel (3:19-23), Chananya, Mishael, and Azarya all survived the fiery furnace of Nebuchadnetzer. These servants wanted to see if the same would hold true for Mordechai, as well.
Me’am Loez notes that it is interesting that they reported Mordechai’s conduct to Haman rather than to the king who issued the decree Mordechai is ignoring. Had they told the king, Mordechai would have been summarily executed. Informing Haman allowed the possibility that Mordechai would have been tortured until he submitted. Psychologically, these servants could then feel better about themselves and their behavior. This, coupled with preserving the status quo, is a powerful motivator. From their selfish perspective, it would certainly be better than turning Mordechai into a martyr.