The Greek and Persian historians following the time of Xerxes the Great (= Ester's Ahasuerus) were not kind to him. They generally painted him as a rather dull, slow-witted character. How much of this is accurate is highly debated. It may have been because of the two great events that marked his reign (486 - 465 BC) made him a man of some ridicule:
- The complete failure of the Greek invasion (480 BC) and the vast wasted cost of the campaign
- He was assassinated by Artabanus, a most ignominious end to someone who styled himself as "Great"
The book of Esther is more neutral about Xerxes's character simply painting him as overly trusting of the scheming Haman.
When Haman discusses the Jewish genocide with the Ahasuerus, he does so in deliberately veiled language ("certain people" Est 3:8) and the king makes very few inquiries about the details but leaves it all to Haman by giving him free reign; even providing the royal signet ring (V10).
It was Haman who organised and composed the royal decrees and dispatches, something that was only possible, in ignorance of the king, because Haman possessed the royal signate ring to authorize such dispatches. It is clear that, at least in this instance, the king was little interested in the details of government and delegated it to Haman.
It should also be recalled that neither Haman nor the king were aware that Esther was Jewish; equally, the king was ignorant that Mordecai was Jewish, although Haman knew.
Therefore, the circumstances here strongly suggest that Ahasuerus was quite ignorant of the details of Haman's plans. Had he been aware, it is almost certain that he would have prevented them from being hatched. This is confirmed by the kings actions in Esther 7 - when the king discovered what had happened, Haman was executed.