We don't know much about Marshal Herrick, except that he's a marshal and his name is Herrick. He does show some kindness to Sarah Good by giving her a swig of his hard cider not long before she's about to be executed. He seems like an average guy just doing his job.
Unfortunately, his job aids and abets the execution of innocent victims. It may be that Miller chose to include Herrick as an example of how sometimes just doing what you're told can have massively dire consequences for your overall society. (Red Scare, anyone?)