S. R. Hirsch (on 21:6, 1989, p. 380) and N. Leibowitz (1982, pp. 260-263) argue that verb for the word send in 21:6 means not that G-d specifically sent snakes, but that G-d removed his protection of the people in the desert. With this idea, there were always snakes in the desert, and only now were they able to attack the Jewish people. Also, with this idea, the snakes were not all over the camp, but 21:6 records that many people died. Maybe, the word many is relative, and the people thought that many people were dying from the snakes. Also, Devarim 8:15 records that not only were there snakes in the desert but also scorpions, so if G-d removed the people's protection, then they also should have been bitten by the scorpions. However, maybe G-d only partially removed the protection from the people, and they were still protected from scorpions.
Why did G-d send snakes, and not a plague as occurs by 17:11 and 25:9? Maybe the snakes had a double symbolism of the desert and of Egypt and more specifically of Pharaoh, who might have had an image of a snake on his crown. Milgrom (1990, p. 174) notes that the word seraph (21:6,8) connotes winged snake, which is similar to the winged Egyptian uraeus (cobra). This double symbolism could correspond to the two complaints, one, about leaving Egypt and two, about the mahn in the desert.
A different idea is that the snakes were because the people were complaining that their lives were bad since they had to eat the mahn, and hence they were punished with snakes that bit and burnt them, and then they were truly suffering.
Or, maybe the punishment of the snakes was because the cure was to make a copper snake, 21:9, but then the question is what is the message of the copper snake?
After people died from the snakes, the people said they had sinned, 21:7, and this was a form of repentance, which marks the second generation as being more worthy to enter the land of Israel than the first generation. One might have thought that this repentance would have been sufficient to end the attack of the snakes, but it was not. 21:8 records that in response to the people’s statement, G-d said to Moshe to make a seraf, and put it on a pole, which the people would be able to look at and survive. 21:9 then records that Moshe made a copper snake, and people who were bitten by a snake could look at the copper snake and live.
Rashi (on 21:9) notes that Moshe was not told to make a copper snake, but that he did this on his own since the word for copper, nechoshet, is similar to the word for snake, nachash. This is a nice literary idea, and also the sounds relate to the word biting, nashach in 21:6,8,9. A different idea is that Moshe learned from the incense pans used by the 250 “men of renown” in the rebellion of Korah, that they were made from copper and put on the altar afterwards, 17:1-4. Another possibility is that the word seraf, which today we do not know its meaning, is related to copper. Or, maybe copper was the most accessible and easy to use item to make the snake.
One unusual point of this incident is that making the artificial snake surely took some time, which might have meant that some people died during the time it was being made. This differs from the case of Aharon taking the incense to stop the plague that Moshe told him to go fast and Aharon ran with the incense, 17:11,12. Maybe the reason for the delay in the people’s salvation was because the attack of the snakes was because the people became inpatient when they had to march around Edom, 21:4, and hence when they were being saved, they had to learn to be patient.
The incident ends with the people being able to be saved if they were bitten by a snake, but it is not recorded that G-d stopped the snakes from attacking the people. It could be that for the rest of the time the people were in the desert, the people would need to look at the snake Moshe made if they got bitten. Thus, following N. Leibowitz’s understanding that G-d allowed the snakes to attack the people, this change was not revoked. This would be a partial step to learning to live normally, as people had to worry and make an effort not to be bitten by a snake, although being healed by seeing the snake on the pole was a miracle.
Why did the people have to look specifically at a snake? The looking was because the second generation had to participate in their repentance, like by Pinhas, 25:4-9, but why did they have to look at a snake? Is it just to show that the antidote comes from the poison?
The Mishnah (Rosh Hashanah 3:8, quoted by Rashi on 21:8) wonders how this metal snake could have healed the person, and it answers that the person was looking towards G-d in the heavens. Yet, if this was the point, then there was no need for there to be a snake on the pole since the people just had to look up to the heavens. Also, unless this pole was very high (three meters high?), the people were not looking up, but looking more horizontally than vertically.
The Mishnah (Rosh Hashanah 3:8, quoted by Rashi on 21:8) wonders how this metal snake could have healed the person, and it answers that the person was looking towards G-d in the heavens. Yet, if this was the point, then there was no need for there to be a snake on the pole since the people just had to look up to the heavens. Also, unless this pole was very high (three meters high?), the people were not looking up, but looking more horizontally than vertically.
A different answer following the idea that the snake symbolized the desert and Egypt is that the people had to look at the snake, to take an honest look at what was life in Egypt. If they took an honest look, then they would remember that they had been slaves and suffered in Egypt, and hence they should be grateful to G-d for taking them out of Egypt even if they did not like the mahn and even if they would not get into the land of Israel. Also, by looking at the snake they would realize what life in the desert is really like, and how G-d had protected them in the desert. This gratitude to G-d would save them from the snakes which was a punishment for their ingratitude towards G-d.
A different idea is that the image of the metal snake on the pole meant that there was a snake above them, maybe at the level of their heads, as well as snakes below them. The snake above them counters the curse said to the snake, but blessing to Adam and Havva in the Garden of Eden, that people would be above the snakes, Bereshit 3:15. Seeing this artificial snake would be a warning to the people that even though they were supposed to go to the land of Israel, if they kept complaining then they would not get there, just like the snake being above them was symbolizing that the blessing to Adam and Havva about snakes could be rescinded. With this approach the lesson to the people from looking at the snake above them was that blessings are conditional. Maybe the people understood this lesson, and we do not hear of the people again asking to go back to Egypt.
Hirsch, S. R. (1808-1888), 1989, The Pentateuch, rendered into English by Isaac Levy, second edition, Gateshead: Judaica Press.
Leibowitz, Nehama (1905-1997), 1982, Studies in Bemidbar, translated by Aryeh Newman, Jerusalem: The World Zionist Organization.