The reason Purim is celebrated for two days in a "walled city" is that the Megilat Esther says the Jews of the city of Shushan (the capital) needed/took an extra day to fight for their survival. My question is: Why did it take longer in Shushan than any other place? Were the citizens of Shushan less ethical/moral or more hateful? How do we know? Is that true for larger cities in general, then or today?
I don't agree with the premise of the question: "...the Megilat Esther says the Jews of the city of Shushan (the capital) needed/took an extra day to fight for their survival. "
Initially, after Haman was killed, permission was granted for the Jews to defend themselves on the 13th Adar - and to kill their enemies.
For some reason, Esther asked for permission - (late on the 13th) - for the Jews to continue killing their enemies also on the 14th.
Due to lack of fast-inter-city communication it was pointless for her to ask permission for areas beyond Shushan.
It doesn't say it was to "fight for their survival". It says (Esther 9:13):
יג: וַתֹּאמֶר אֶסְתֵּר אִם עַל הַמֶּלֶךְ טוֹב יִנָּתֵן גַּם מָחָר לַיְּהוּדִים אֲשֶׁר בְּשׁוּשָׁן לַעֲשׂוֹת כְּדָת הַיּוֹם וְאֵת עֲשֶׂרֶת בְּנֵי הָמָן יִתְלוּ עַל הָעֵץ:
She asked for permission to continue doing on the morrow what they had done that day, and to hang the 10 sons of Haman (that were killed that day).
It seems to me that the issue was to uproot any supporters of Haman left in Shushan, especially within the government\palace. This is why she had Haman's son's hanged, and the people killed were probably affiliated with Haman's family. The purpose was to solidify Mordechai's position and to make sure the Jews would be secure for years to come.