As Rashi said above, Haman’s motivation was fear of retribution without the king’s explicit permission.
The reason for this is, ironically, his own participation in creating a law that women have to listen to their husbands (Esther 1:22). As the Malbim wrote there, the necessity to sign such a truism into law put into question any other otherwise immutable ideas. Even a minister being disrespected did not act without the king’s command.
According to Rebbetzin Heller, the reason Haman restrains himself is that a good retaliation requires careful planning.
The Baal HaTurim (on Bireishis 43:31) points out that the word vayisapek (“and he restrained himself”) is used only twice in TaNaCh. The first is where Yosef restrains himself from revealing himself to his brothers. Just as in the Yosef story, the predicament was precarious but ended positively, so too in this story, self-restraint leads to national joy for the Jews. The Ramban’s definition (on Vayikra 19:2) of becoming holy is to restrain oneself and not to over-indulge.