Richard Rich is the secret linchpin of A Man for All Seasons. Cromwell might be the one leading the charge to take Thomas More down, but Rich is the one who hammers the final nail in our hero's coffin.
Remember, the play starts with More telling Rich that he doesn't belong in politics. According to More, the guy's super smart, so he should probably be a teacher or something, but he seems to lack the moral fortitude required to steer clear of the temptations that come with political office. Just listen to More tell it:
MORE: But, Richard, in office they offer you all sort of things. I was once offered a whole village, with a mill, and a manor house, and heaven knows what else. (1.66)
As we know, Rich epically fails the bribery test: he turns against More after Cromwell offers him a series of cushy but unglamorous gigs. This decision is met with an increasing amount of guilt on Rich's part, but that doesn't stop him from lying under oath in order to get More beheaded—and, crucially, to get just a little more power for himself. Even then, however, More sees through Rich's facade and twists the knife:
MORE: In good faith, Rich, I am sorrier for your perjury than my peril. (2.754)
See, Rich isn't some evil, power-hungry maniac like Cromwell—he's simply weak. Weak and ambitious—a bad combination. And Rich knows it. Ultimately, this weakness shows us that you don't just have to worry about the bad guys; you also have to worry about the unremarkable people desperate to grab their own little piece of the pie, by any means necessary.