The Scofield Bible Commentary, by Cyrus Ingerson Scofield, [1917], at sacred-texts.com
rod
"Sign of the rod" means "power", (Psa 110:2); (Psa 2:9); (Rev 2:27). It was Moses' shepherd's crook, the tool of his calling. Cast down, it became a serpent; taken up in faith, it became "the rod of God"; (Exo 4:20); (Exo 7:12).
(See Scofield) - (Exo 7:12)
Put now thine hand
The sign of leprosy. The heart ("bosom") stands for what we are, the hand for what we do. What we are, that ultimately we do. It is a sign of (Luk 6:43-45). The two signs, rod and hand, speak of preparation for service:
(1) consecration -- our capacity taken up for God;
(2) the hand that holds the rod of God's power must be a cleansed hand swayed by a new heart.
Aaron the Levite
(Cf) (Exo 28:1).
(See Scofield) - (Exo 28:1).
harden his heart
(Cf) (Exo 8:15); (Exo 8:32); (Exo 9:34). In the face of the righteous demand of Jehovah and of the tremendous attestations by miracle that He was indeed God, and that Moses and Aaron were His representatives, Pharaoh "hardened his heart." Instrumentally God hardened Pharaoh's heart by forcing him to an issue against which he hardened his own heart in refusal. Light rejected, rightful obedience refused, inevitably hardens conscience and heart. (Rom 9:17-24).
kill him
(Cf) (Gen 17:14) The context (Gen 4:25) interprets v. 25. Moses was forgetful of the very foundation sign of Israel's covenant relation to Jehovah. On the eve of delivering Israel he was thus reminded that without circumcision an Israelite was cut off from the covenant. (Jos 5:3-9).