1)
Why did they say "Ish Mitzri"?
1.
R. Bechayei, citing Shemos Rabah 1:32: Moshe told them, 'The Egyptian that I killed (caused that I fled Egypt and came to here. He) saved you!'
2.
Pesikta Zutresa: He was born in Egypt.
3.
Bechor Shor: From his clothes and language, they recognized that he is from Egypt.
