"On that night, the king’s sleep was disturbed, and he ordered to bring the book of records, the chronicles, and they were read before the king." (Esther 6:1)
Our Sages say in the Gemara (Megillah 15b) that the thought crossed King Achashverosh’s mind that perhaps Haman was plotting to have him killed. At first glance, this seems very strange: from where did such a suspicion arise, if Haman was his most faithful and closest servant?
It can be explained according to what our Sages teach (Gittin 56b): "Whoever oppresses Israel ends up becoming a leader." The reason for this is clarified in the Tosafot (Chagigah 13b): so that it is not said that Hakadosh Baruch Hu delivered Israel into the hands of a despicable nation.
Therefore, Achashverosh feared that, since Haman desired to exterminate Israel, it would not suffice for him to be considered a "head" only by his wealth or power, but rather that this would lead to Achashverosh himself dying and Haman ascending to the throne in his place. In that way, Haman would become a true and worthy "head" or "chief," and then — according to that reasoning — Israel would be destroyed through him.
This thought was what unsettled Achashverosh: for Haman’s plan to exterminate the Jews to succeed, Haman had to become a "head," because only a "head" can strike against the People of Israel; thus, this implied that he (Achashverosh) had to end up dying so that Haman could rise to power.
(Zera Shimshon, Esther, Art. 4)
