The Plague of Being Unable to See One’s Friend:
Sefer Emes L’Yaakov (written by Rav Yaakov Emanuel Koppel zt”l, son-in-law of the Divrei Yisroel of Modzhitz zy”a) states that we see from this pasuk that one must always look to see what his friend needs and attempt to help him. We see that the plague of darkness was that “a man could not see his brother.” Not being able to see one’s brethren’s needs is a makkah.
The mesorah connects this verse that says that “a man could not see his brothers” to the verse that forbids persecuting one’s brother (Vayikroh 25:14), the verse that says that one should perform kindness to his brothers (Zechariah 7:9), and the verse after the sin of the eigel (Shemos 32:27), in which Moshe said: “Let every man kill his brothers.”
The meaning of this is that a man is forbidden to persecute his brother. Rather, he should show his brother kindness because if one fails to see his brother he may, Heaven forbid, end up killing him.
