Haman persuaded Ahashverosh to sign a decree enabling the genocide of the Jews in the Persian empire. This much is clear from the text. But in between Esther's parties, Haman constructs a gallows 50 cubits tall, at his own home, specifically to hang Mordechai. I'm a little befuddled by how he could think he could get away with this. True, he lurked in the king's courtyard that night, probably rehearsing his speech and/or waiting for the king to wake up, so he could request permission for Mordechai's hanging. But wouldn't Ahashverosh find it a little bit odd that Haman had already built the gallows at his own home to hang Mordechai?
Under what kind of system of laws were the Persians operating that Haman could conceive of this as somehow legitimate - or legitimate enough to avoid some kind of inquiry by the king's court into what the heck he had planned and for how long?