From Wikipedia:

The Politiques were horrified but many Catholics inside and outside France regarded the massacres, at least initially, as deliverance from an imminent Huguenot coup d'etat. The severed head of Coligny was apparently dispatched to Pope Gregory XIII, though it got no further than Lyons, and Pope Gregory XIII sent the king a Golden Rose. The Pope ordered a Te Deum to be sung as a special thanksgiving (a practice continued for many years after) and had a medal struck with the motto Ugonottorum strages 1572 (Latin for "slaughter of the Huguenots") showing an angel bearing a cross and sword next to slaughtered Protestants.

Pope Gregory XIII also commissioned the artist Giorgio Vasari to paint three frescos in the Sala Regia depicting the wounding of Coligny, his death, and Charles IX before Parliament, matching ones on the defeat of the Turks at the Battle of Lepanto (1571). "The massacre was interpreted as an act of divine retribution; Coligny was considered a threat to Christendom and thus Pope Gregory XIII designated 11 September 1572 as a joint commemoration of the Battle of Lepanto and the massacre of the Huguenots."

On hearing of the slaughter, Philip II of Spain “laughed for the only time on record”. In Paris, the poet Jean-Antoine de Baïf, founder of the Academie de Musique et de Poésie, wrote a sonnet extravagantly praising the killings. On the other hand, the Holy Roman Emperor, Maximilian II, King Charles's father-in-law, was sickened, describing the massacre as "shameful". Moderate French Catholics also began to wonder whether religious uniformity was worth the price of such bloodshed and they began to swell the ranks of a movement, the Politiques, which placed national unity above sectarian interests.

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Papal Reaction

THE MASS MURDER RECEIVES OFFICIAL CHURCH BLESSING

Upon hearing of the bloodshed in Paris, Pope Gregory XIII celebrated by declaring a jubilee day of public thanksgiving.  Guns were fired in salute and the Pope ordered the striking of a commemorative medallion.  Subsequently, Gregory XIII also commissioned a mural by Giorgio Vasari to hang in the Vatican depicting the wondrous St. Bartholomew's Day Massacre.

Various scholars have depicted the Pope's reaction as follows:

"The news of the bloody deed was received with unbounded joy by the pope at the Vatican. The Cardinal of Lorraine presented the messenger who brought the news to Rome with a thousand pieces of gold and exclaimed that the King's heart had been filled with a sudden inspiration from God when he ordered the massacre."

"The pope and his Cardinals proceeded at once to the High Altar, after the dispatches from Paris had been read in Conclave, to offer thanks for 'the great blessing which Heaven vouchsafed to the Roman See and to all Christendom. Salvoes of artillery thundered at nightfall from the ramparts of St. Angelo; the streets were illuminated; and no victory ever achieved by the arms of the Pontificate elicited more tokens of festivity. The pope also, as if resolved that an indestructible evidence of the perversion of moral feeling which Fanaticism necessarily generates should be transmitted to posterity, gave orders for the execution of a commemorative Medal."

"Through the streets of the Eternal City swept, in the full blaze of Pontifical pomp, Gregory and his attendant train of cardinals, bishops and monks, to the Church of St. Mark, there to offer up prayers and thanksgivings to the God of heaven for His great blessing to the See of Rome and the Roman Catholic Church. Over the portico of the church was hung a cloth of purple, on which was a Latin inscription most elegantly embroidered in letters of gold, in which it was distinctly stated that the massacre had occurred after 'counsels had been given.' On the following day the Pontiff went in procession to the Church of Minerva, where, after mass, a jubilee was published to all Christendom, 'that they might thank God for the slaughter of the enemies of the Church, lately executed in France.' A third time did the pope go in procession, with his cardinals and all the foreign ambassadors then resident at his court, and after mass in the Church of St. Louis, he accepted homage from the Cardinal of Lorraine, and thanks in the name of the King of France, for the counsel and help he had given him by his prayers, of which he had found the most wonderful effects." 

Pope Gregory was jubilant when news of the massacre reached the Vatican!!

When news of the Massacre reached the Vatican, there was jubilation! Cannons roared—bells rung—and a special commemorative medal was struck—to honor the occasion! The Pope commissioned Italian artist Vasari to paint a mural of the massacre—which still hangs in the Vatican!!

The slaughter of over 100,000 unarmed French Protestants (Huguenots) within three weeks in 1572. Two years earlier the Huguenots were granted freedom of worship within certain localities in France. The slaughter began in Paris, where leaders of the Huguenots were gathered in support of the king of France against a likely war with Spain. The king’s mother, Catherine de Medici, was related to the Papal Nuncio (ambassador) to France. She asked him to tell the Pope that “he would soon see the vengeance that she and the king would visit” on the Huguenots. She convinced the king to kill the assembled Protestant leaders. The royal troops then began a slaughter which spread to the outlying towns of France. Upon hearing of the massacre, Pope Gregory XIII ordered a special jubilee in celebration. He commissioned that a medal be struck showing an exterminating angel smiting the Huguenots with his sword.