Julius Caesar and Otello (the version of Othello by Giuseppe Verdi and his librettist Arrigo Boito): These are the texts that framed the final remarks of federal Judge George A. O’Toole Jr. in the case of Dzhokhar Tsarnaev, convicted last month of the Boston Marathon killings.

The Tsarnaev case moved Judge O’Toole to reach for the kind of precedent that not law but literature makes available.

“One of Shakespeare’s characters observes: ‘The evil that men do lives after them. The good is oft interred with their bones.’ So it will be for Dzhokhar Tsarnaev.”

With these words, Judge O’Toole took the language out of the mouth of Mark Antony and transformed it into a moral sentiment applicable to the gravity of Tsarnaev’s conviction.

As one of my email correspondents pointed out, Judge O’Toole might have found in a canonical American authority — Jefferson? Franklin? Lincoln? — words about posterity and evil. But the judge felt that Shakespeare, not a Founding Father or Honest Abe, provided the words needed.

Shakespeareans roll their eyes at citations of this kind — and there are many of them — in which a turn of phrase (“Neither a borrower nor a lender be”) becomes a verity, not a line of dialogue in a particular dramatic situation.

In this instance, the difficulty with the quotation is that Mark Antony really does come to praise Caesar, to wind the crowd up and turn them against the conspirators. Caesar’s good is exactly what he doesn’t want interred with Caesar’s bones.

As I’ve been writing a book on Shakespeare and opera, I was especially struck by the judge’s second citation. I quote him here:

“In Verdi’s opera Otello, the evil Iago tries to justify his malice. ‘Credo in un Dio crudel,’ he sings. ‘I believe in a cruel god.’ Surely someone who believes that God smiles on and rewards the deliberate killing and maiming of innocents believes in a cruel god. That is not, it cannot be, the god of Islam. Anyone who has been led to believe otherwise has been maliciously and woefully deceived.”

I wondered if the judge thought that Tsarnaev had ever heard — or even heard of — Verdi’s Otello.

The monologue Judge O’Toole cites here begins “Credo in un Dio crudel che m’ha creato simil a se, e che in ira io nomo.” I believe in a cruel god who created me in his image, and whom I name in anger.

Clearly an opera fan, Judge O’Toole would have known that Iago’s “Credo” doesn’t appear in Shakespeare. As Verdi’s librettist, Boito felt the need for a dramatic statement on Iago’s part, and so he added this text to the opera’s stripped-down version of the play. The “Credo” might not be by the Bard, but Verdi thought the added passage was “pure Shakespeare.”

Pure Shakespeare. Verdi’s idea of “pure Shakespeare” was a passage that, while invented, captured the essence of the playwright’s mind and language.

But the judge’s use of a famous line from Julius Caesar, as well as (in a more complicated way) a famous line from Verdi’s Otello, suggests a belief in a different kind of “pure Shakespeare” — Shakespeare the moralist, Shakespeare the universal truth-teller.

This isn’t, in fact, a very literary Shakespeare, at least as those who study Shakespeare and his texts see him. Instead it’s a Shakespeare of parts, a craftsman whose language we can return to on a sentence-by-sentence basis, separated out from context.

That Shakespeare is endlessly quotable. Just take a look at Amazon.com’s offerings for “quotable Shakespeare” — a book or more a year appears offering collections, selections, A-to-Z’s, and calendars replete with many a Shakespearean mot of the bon variety.

So we’re caught in a citational dilemma. Can we advise, unironically and out of context, that “the quality of mercy is not strained” even if we know that The Merchant of Venice is a deeply complicated play sometimes masquerading as almost a comedy?

Or that one should “to one’s own self be true” even knowing that Polonius, who speaks these words in Hamlet, is a bit of a fool?

Should there be such a thing as Shakespeare-quotation best practice?

Judge O’Toole was moved by sentences from Shakespeare and — in the case of Verdi — “from” Shakespeare. Maybe he wanted to trace a connection between two kinds of sentences.

Or maybe, gazing one last time from bench to defendant, he simply needed help in framing the gap between language and act.