This is just one of many descriptors in Jesus’ Epic Tirade* for the “scribes and Pharisees”—the religious leaders of Jesus’ day.
Being called a snake is unpleasant at the best of times, even without considering the recurring symbolism throughout the Bible of Satan, the Great Deceiver, as a serpent…
Jesus isn’t actually the first to use this slur at the Pharisees, nor is this the only time He does so. In Matthew 3:7, John the Baptist shuts down some religious types with “You brood of vipers!” And in Matthew 12:34, Jesus responds to the Pharisees’ allegations by saying, amongst other things, “You brood of vipers, how can you who are evil say anything good? For the mouth speaks what the heart is full of.”
* As opposed to all His other tirades aimed at the scribes and Pharisees. There are many, but this one is particularly long and scathing.