It is interesting that YHWH asks Moses to make a "seraph" in the story and not exactly a snake as translations usually say. The word "seraph" comes from the word for "fire," and these divine creatures are often connected with fire (see, for example, the famous story of the "seraphim" that surround and serve the altar in Is. 6). In response to YHWH's request for a seraph, Moses instead makes a copper viper. Whether Moses has in fact done what YHWH asked of him, the result of his efforts is that the snake-bit Israelites are saved by a glance at the copper viper pole, now made visible to them.
Well, that's the story. Are we now more enlightened about it now that we have looked more closely at it? Perhaps. We can now see that it is a part of that tradition of the wilderness where the Israelites are impatient grumblers thoroughly dissatisfied with Moses and with YHWH. Both Moses and YHWH become in their turn angry and frustrated with these ingrates and move to punish them in many and various ways. That seems clear enough.
But is that all? What about the magic of the copper viper pole? I am inclined to leave it as is, a piece of ancient necromancy best left in the distant past. And though John's gospel lifted it up allegorically to refer to Jesus' saving power, I remain suspicious that such textual use can finally be useful to us in our time. We need no magic poles to teach us that Jesus brings to us snake-bitten moderns a power and grace that only he can provide. Numbers 21:4-9 is quaint but less than efficacious as a necessary element in my Lenten journey. I leave the story and its fiery snakes far behind with few regrets.