Megillat Esther says that Mordecai advised Esther not to tell anybody at court she was Jewish. But when Esther finally tells the king, after Haman is executed, his reaction shows no negative feelings towards the Jews -- he allows them to fight back and makes Mordecai "haYehudi" his chief advisor. So was Mordecai just being over-cautious? Would the whole problem have been avoided if Esther had told the king she was Jewish from day one? Anything on this from the commentators?
Let us assume that Ahashverosh didn't know Ester is Jewish (I seriously doubt it it but it is a different question.) My answer is a little Drash though, I can not answer it on Pshat.
THe key point is to decide what side Ah. was through the first part of the Aggadah. According to AriZ"L he was all bad and he was on Haman's side from the beginning, realizing that Hashem failed Jews and they will not revive and build the Mikdash etc.
So his idea of reigning the kingdom of Tumah (as opposite of Malchut deKedushah) could only be implemented by marrying a symbol of external beauty that does not come from Kedushah (as we see with our fore-mothers that were exceptionally beautiful because of their inner traits) but from Tumah, i.g. the gentile nations. If he knew that Ester's beauty comes from the beauty of her Jewish soul, the whole plot would be undermined.
When Ahashverosh revealed Ester's nationality, he immediately realized that his very assumptions of Hashem failing His promises to Jews were wrong and the best he can do is to subdue himself to the "winning side" - the good guys!
That's in short.