Rav Dovid Feinstein writes the king should be expected to ask advice before proclaiming a major ruling. In a case like this, though, he also needed a sense of what was best – not just what was right. This required both knowledge and justice. The Midrash (Esther Rabbah 4:1) points out that Achashverosh, though not a righteous man by any stretch of the imagination, still had a “mida tova” (good characteristic) of seeking advice before making big decisions. Rabbi Eliezer Ginzburg writes in Ginzei Hamelech that “justice” implies going beyond the letter of the law.