The scrivener (no relationship to Herman Melville's Bartleby) is the guy whose job is to neatly copy Hastings' indictment. The scrivener knows Hastings' death is shady, and he's outraged that Richard and Buckingham have tried to justify executing him without a trial. So what? Well, his outrage gives voice to the concerns of London's citizens, who aren't happy about the corrupt government.
Here's a good world the while! Who is so gross
That cannot see this palpable device?
Yet who's so bold but says he sees it not?
Bad is the world; and all will come to nought,
When such ill dealing must be seen in thought. (3.6.1)
In other words, Richard may be a great manipulator, but he's not really fooling anyone in the kingdom anymore.