Ahasuerus was a hypocrite. Not in the casual sense of the word, but in the specific, devastating way that Esther Rabbah defines it: a ruler who kills his wife for his friend, and then kills his friend for his wife.
Rabbi Yuda son of Rabbi Simon opened with a verse from Job: "From a hypocritical person's rule; from snares of the people" (Job 34:30). Rabbi Yohanan read this as cause and effect. When a king is a hypocrite and rules the people, the word "snares" (mimokeshei) connects to "stubbornness" (kashyuteihen). The people get the king they deserve when they refuse to do the will of their Creator. Reish Lakish took a more dramatic approach: it would be better for people to sprout wings and fly through the air than to live under a hypocritical king.
The midrash (rabbinic interpretive commentary) then names Ahasuerus as the prime example. He executed Vashti, his queen, because Haman and the advisors urged him to. Later, he executed Haman because Esther, his new queen, pleaded with him. He killed his wife because of his friend. Then he killed his friend because of his wife. This is not justice. This is a man who bends to whoever spoke last.
Abba Oriyan of Sidon then listed a chain of social decay, taught in the name of Rabban Gamliel. When lying judges increase, lying witnesses increase. When informants multiply, the people's property is plundered. When brazenness rises, glory and honor are stripped from the nation. And when undefined's beloved children anger their Father in Heaven, He sets over them a hypocritical king who exacts retribution from them. That king was Ahasuerus. When everyone saw that it was so, they began screaming: "Woe!" And so the story begins: "It was during the days of Ahasuerus" (Esther 1:1).