One may smile, and smile, and be a villain
Hamlet:
O most pernicious woman!
O villain, villain, smiling, damnèd villain!
My tables—meet it is I set it down
That one may smile, and smile, and be a villain—
At least I am sure it may be so in Denmark.
Hamlet has just encountered for the first time the ghost of his father, who was, as he tells his son, poisoned by his own brother Claudius (the "smiling, damned villain"), Claudius then grabbed both the old king's crown and his queen—Hamlet's mother, that "pernicious woman."
The ghost, like Hamlet, assesses the queen's remarriage as "luxury and damned incest" (line 83). But he asks Hamlet to let heaven punish that crime; his son's job is to take revenge on Claudius. And though Hamlet's spite is directed first at his mother [see FRAILTY, THY NAME IS WOMAN], here he lingers lovingly on the usurper's treachery.
We've briefly encountered the new king's unctuous rhetoric in earlier scenes [see MORE THAN KIN AND LESS THAN KIND]. Hamlet hated it from the start, but now that he knows about Claudius's fratricide, distaste has become perverse fascination. He pulls out his "tables" (a writing tablet) to note down that one may "smile, and smile, and be a villain." In Shakespeare's previous tragedy, Julius Caesar, the young Octavius delivered the working version of Hamlet's aphorism: "And some that smile have in their hearts, I fear,/ Millions of mischiefs" (Act 4, scene 1, 50–51).