When Jesus Christ and St. John the Baptist call the Pharisees a brood of vipers, they are not slandering, for this is a violation the the Law of God. They are warning the peoples gathered that they are children Satan as St. Paul the Apostle writes later in his letters.
It is a wise and manly rebuke with an element of metaphor to speak to the masses concerning the deceivery the Pharisees are engaging in. Without such metaphor, they’d have to be far more explicit in these Pharisees are taught and birthed by Satan. I believe the former statement is far more benign and also effective than the latter.
If truth is spoken, it can be harmful to reputation, however it cannot be considered slander when it is spoken rightly and in wisdom for the sake of the betterment of humanity.