Why does the Torah need to inform us what those who feared Hashem did; and, in the next Pasuk (9:21), what those who did not fear Him did?
Seforno (also see Rashi to 9:10): The Torah is referring to what the various Egyptians did by the plague of Dever. 1 What the Pasuk means is that following the reaction of the those who feared Him then, and how those who didn't fear Him suffered the consequences, Hashem now instructed the people to gather them in now, lest they suffer the same fate.
Targum Yonasan: Because "those who feared Hashem" refers predominantly to Iyov, 2 and "those who didn't," to Bil'am 3 - two of Pharaoh's three counselors, who played major roles in influencing his treatment of Yisrael. 4
Perush Yonasan: See Ba'al ha'Turim (to 9:17).
It is safe to assume that the Pasuk does not hint at Yisro (who had been the third counselor, refer to Bamidbar 24:21:2:1*), because he had already fled, and no longer lived in Egypt; in which case he was not subject to the plagues.
This verse only tells us how Pharaoh's servants reacted; why does the Torah not mention what the Egyptian people at large did?
Oznayim la'Torah: It appears that Pharaoh did not publicize the impending plague of hail, for fear that they would panic - and it was only his servants who were present when the warning was issued who were aware of it. 1
Oznayim la'Torah: Interestingly, 'the ice had begun to melt' and some of his servants already feared Hashem and took their animals inside. See Oznayim la'Torah.
