Or HaChaim on Shemos 2:11

English: 2,11 איש עברי מאח ;יו , a Hebrew man, one of his brethren. The Torah makes a point of adding "one of his brethren." This is the Torah's way of hinting that Moshe recognised that the individual in question was one of the righteous Israelites. There were at that time both righteous and wicked Israelites. It is worthwhile reading how the Mechilta 12,26 interprets the meaning of Exodus 13,18 that the Israelites marched out of Egypt חמושים , reduced to one fifth of their original number. According to the Mechilta only one in every five Israelites took part in the Exodus. The other 80% of the Israelites (the wicked ones) died during the plague of darkness so as not to publicise that fact and allow the Egyptians to gloat. The two Jews who are described as Jews fighting amongst themselves in verse thirteen were not described as "Moshe' brethren;" this proves that they were wicked Jews; according to Shemot Rabbah 1,29 they were Datan and Aviram of whom we hear more in פרשת קרח .