The words of John the Baptist, addressed to Pharisees who were, in general, deeply religious and pious, sound somewhat strange. And the point is not only that he calls them "brood of vipers," but also his words about their desire to avoid the wrath approaching them, by which he clearly means the wrath of God.
Much is explained here by the religious psychology that most of those Pharisees who did come to John for washing were guided by, though there were hardly many such people, since most Pharisees were skeptical of John's words and activity. Apparently they simply believed that there can never be too many purifying washings, and that one more would not be superfluous in any case.
John called for conversion and repentance, being sure that everyone without exception needed this, as well as the corresponding purifying washing, that among the people of God there were none ready to meet the Messiah, that there were no pure ones who could go out to meet Him with a calm conscience. John partly inherited such a view from the Essenes, with whom he was likely closely connected for some time, but to a great extent it was the consequence of the revelation John himself received, the revelation that made him go out to preach.
Most Pharisees did not agree with the prophet, being sure that they at least were ready to meet the Messiah: after all, they had devoted their whole lives to preparing for that meeting, and purity, spiritual, moral, and ritual, occupied the chief place in their lives. But, as one can see, among the Pharisees there were also some who could at least suppose that everything John said was true.
What if he really was a prophet, a man of God? What if he was right? Then, just in case, it would be better to perform the purification to which he called. In any case it would not be worse, and it could be better if this strange preacher turned out to be right after all. But such a washing was, of course, not at all what John was calling for. Here there was precisely the desire to perform one more purification "just in case," so as to please God with a guarantee. To avoid that very wrath to come. Which, without a real awareness of one's own sinfulness, could not be avoided anyway.
