According to a Midrash, Achashverosh is upset at this point because, in the garden, Achashverosh remembered that Haman (as Memuchan) was responsible for Vashti’s demise (Esther 1:16).
The Dena Pishra adds that Achashverosh was upset that Haman was speaking to Esther behind his back. He even considered that if she pleads for his life, Achashverosh would still not listen to her.
The Yosef Lekach notes that Achashverosh took Haman’s silence as admission of guilt, based on this principle in the Talmud (Yevamos 87b) that people are expected to speak up for themselves when accused unjustly.
The Malbim writes that Achashverosh was angered that Esther seemed included in decree without his expressed agreement. The Malbim adds that this anger created an unsafe environment in the palace, despite its providing political and legal sanctuary.
The Ben Ish Chai writes that the Talmud (Shabbos 33a) teaches that dropsy and its attending discoloration affect people who are guilty of immorality. Thus, Achashverosh suspected Haman of immoral acts due to his face coloring.
R’ Moshe David Valle notes that Achashverosh could only think immorality was on Haman’s mind at a time like this if he so was inspired by H-Shem.
Perhaps some insight may be gained on this topic from the Talmud’s (Horiyos 10b) understanding of the story (Shoftim 4:17-22) between Sisera and Yael. There, Sisera is in mortal danger, and yet is easily seduced by Yael. Violence and immorality sometimes go together.
However, according to Midrash Shmuel, Achashverosh did not really think anything immoral was happening. In fact, he did not even accuse Haman of anything like that. Rather, H-Shem had the words come out of the king’s mouth to make Haman more nervous.
The Vilna Gaon writes that Achashverosh simply thought Haman intended Esther harm. Perhaps, this anger was pretense, and was Achashverosh’s method for ridding himself of Haman in the most politically expedient fashion. Interestingly, none of these answers explaining Achashverosh’s anger need be exclusive; the combined reasons create a massive, unappeasable anger that justifies the king’s next act.