Understand the literal history of tarring and feathering to grasp the magnitude of the metaphor. Mentioning they'd tar and feather somebody in historical fiction evokes a brutal image.
Pro Tip 2/3
Severity
Recognize that using this phrase suggests extreme disapproval or punishment. By saying they'd tar and feather somebody, he indicated how unacceptable the act was.
Pro Tip 3/3
Social Anger
The phrase implies punishment is demanded by the wider community, not just an individual. After the scandal, the community acted like they wanted to tar and feather the official.