Va'yar ish Mitzri make is Ivri... vayach es haMitzri
R' Soloveitchik (quoted by R' Shachter) explained derech derush that Egyptian society projected itself as being an enlightened, liberal society. This attracted a good number of Klal Yisrael, who cast off the old fashioned religion of their fathers in favor of this ethos of civility and tolerance that the culture around them represented. What changed Moshe was seeing an "ish" Mitzri, a cultured individual, someone representative of the tolerant, liberal society around him, striking an "ish" Ivri, a Jew who also was an "ish," someone who had assimilated and adopted those same liberal cultural values. Moshe lashed out at the Mitzri -- here the Torah does not say "ish" Mitzri because the mask was off. Moshe saw this man for what he was -- a barbarian dressed in the trappings of enlightenment much like a wolf wearing sheep's clothing.