Compare the following statements:
Avot 2:14-"Warm yourself before the fire of the sages, but be heedful of their glowing coals for fear that you be burned, for their bite is the bite of a jackal and their sting the sting of a scorpion and their hiss the hiss of a serpent, and all their words are like coals of fire."
Compare also this statement of R' Yochanan bar Nafcha - ואמר רבי יוחנן משום ר' שמעון בן יהוצדק כל תלמיד חכם שאינו נוקם ונוטר כנחש אינו תלמיד חכם - translated (Rodkinson) as "A scholar who is not revengeful and remembers not injuries as a serpent, cannot be called "Talmud Hakham" (a teacher)."
Yet another snake reference (Shabbos 63a) אמר ר' שמעון בן לקיש אם תלמיד חכם נוקם ונוטר כנחש הוא חגריהו על מתניך - Rodkinson's translation "If thy teacher is jealous (for thy welfare) and as spiteful as a serpent (if thou neglect thy studies), carry him on thy shoulders (because from him thou wilt learn)" - The interpretation in the parentheses is some sort of apologetics.
Finally compare the following statement attributed to Jesus: "Ye serpents, ye generation of vipers, how can ye escape the damnation of hell?" (Matthew 23:33)
Jesus seemed to be making a double entendre - he was using the Pharisees self reference to themselves as snakes against them. If he was in truth making a pun then he might not have meant to be quite as derogatory as it would appear.
It is fairly frequent in the midst of the ריתחא דאורייתא to find that the Pharisees use very harsh comments against each other in order to add strength to the argument. Possibly Jesus still believed himself to be a Pharisee carrying on the Pharisaical style of sharp argumentation.