What had gotten into Michelangelo that made him destroy this Pietà—a statue he had brought so far to completion?

The Duomo Pietà or (Deposition) c. 155o       226 cm (89 in),  in the Museo dell’Opera del Duomo, Fl0rence
This photo is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license here.

Vasari, who was interested in knowing, can’t say why. “He did this either because it was hard and full of emery and the chisel often struck sparks from it, or perhaps because his judgment was so severe that he was never content with anything he did [my italics].”

That last reason is a fairly good bet. Michelangelo left unfinished statues wherever he went. And now in old age he was depressed, broody, morose even.
He admitted that he had mutilated the statue on purpose.
Here is what Vasari says he told Cacagni, the sculptor who later tried to repair and finish it:

“The reason…..was the importunity of his servant Urbino who had nagged him every day to finish it; and as well as this a piece had broken off from the arm of the Madonna. And these things, he said, as well as other mishaps including his finding a crack in the marble, had made him so hate the work that he had lost patience and broken it; and he would have smashed it completely had not his servant Antonio persuaded him to give it to someone just as it was.” (Vasari)

Cacagni tried to finish the group and thereby spoiled it for good. We are justified in attributing all the bad, all the errors of this Pietà, to Tiberio Cacagni.

But what if some of them weren’t his? Could Michelangelo himself have made one or two? Vasari, by first giving his own theory as to why Michelangelo destroyed the statue, and THEN giving the several reasons that the master himself offered to Cacagni, seems to imply that he doesn’t believe them. And there ARE one too many of these excuses, in the style of the classic liar. Why had Michelangelo become dissatisfied with the statue in the first place—why had his servant Urbino had to nag him to go on? Who can believe that the hard emery veins that made his hammer spark were reason enough to quit? Or even the alleged crack in the block? He had finished the Louvre Slave though it had a terribly disfiguring sand vein right through the face. It’s true he had abandoned at least one figure—the first version of the Minerva Christ—because of a defect in the block; but he hadn’t worked at it so long as this Pietà. Cacagni repaired the crack and the Madonna’s broken arm: why couldn’t the Master have done the same, distasteful work as that was? It was only a small price, a little humiliation, to pay for the preservation of the marvellous work and all its beauty.

No. Michelangelo might very well have made some aesthetic error, or discovered one. He may have uncovered some flaw in the general design that he hadn’t noticed in his small wax model; and he couldn’t come up with a solution. He may have been trying, as Vasari suspected, to confuse Cacagni—and the rest of the world—with his talk of all the technical problems. It was “his dissatisfaction with everything that he did” that was at the root of his crazy destruction of the Pietà. To preclude any suspicion of just that truth, he destroyed left and right, good and bad; and when he was satisfied that all the clues were obliterated, he gave the statue away with, so to speak, a smile. He knew that from then on, the man who tried to patch it up would take the blame for all the bad, even the bad of the design.

What was so bad about the Pietá anyway?  See The Pietà Michelangelo Destroyed 1

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