Given that A Man for All Seasons is based on real-life history, all you'd have to do is look at Thomas More's Wikipedia page to be spoiled about the execution at the end of the play. Oh, well.
We close this thing out with Richard Rich completing his road to corruption, which provides a perfect frame for his opening conversation with More. Remember that? It's basically More telling Rich that he shouldn't get into politics because he will be tempted by bribery and seems to lack the moral fortitude to reject those bribes. And guess what? Rich completely lacks that moral fortitude, as proven by his decision to lie under oath about More in order to get a cushier gig for himself.
After the axe falls (in complete darkness, unseen by the audience), Common Man returns to give us a brief send-off. This closes out another of the play's frames, as Common Man opened the play and frequently provided pithy commentary between scenes. His semi-ironic closing treatise is no different:
COMMON MAN: [...] It isn't difficult to keep alive, friends—just don't make trouble—or if you must make trouble make the sort of trouble that's expected. (2.795)
In other words, don't rock the boat if you're sailing with powerful people. While we can certainly look at Common Man's closing words in an ironic fashion, they still give proper weight to the very real, very grim reality of Sir Thomas More's downfall.