That seems an extraordinary remark for any serious biographer to make. She would likely committed more atrocities? What atrocity was she accused of commiting in the first place? She was far from the only person responsible for the Seven Years' War, given the keen involvement of France and Russia, and given the fact that Frederick the Great had grabbed Silesia by force without any rationnale except his own desire for more land for Prussia, it's not as if he occupied a higher moral ground than she did. Of course, she didn't behave like a meek, biddable woman in seeking to attack Prussia, always something which frightened eighteenth and nineteenth century biographers in particular, but to accuse her of 'atrocities' in the first place seems pretty much of a stretch. And then to remark that she would possibly have committed more if given her way just piles on the absurdity. This is where I feel genuine history/biography starts to diverge from the path supported by evidence and becomes speculative fiction, and it is always heralded by the sort of remark that "X must have thought" or "X very likely did" without any convincing evidence. I'm very willing to accept that Maria Theresa had poor parenting skills, was manipulative and could be harsh to her children, especially her daughters, and unfairly gave one daughter much greater favours which the daughter in question certainly took full advantage of. But I really don't see the evidence for her being an atrocity-commiting vengeful monster of unbridled grudges and resentments.
The biographer was Paul Tabori, who was Hungarian-British. One could say that due to his heritage, he would've been partial to make excuses for Maria Theresa. Although his biography ("Maria Theresa" in "the Women who made History" series) had some fanciful accounts and incorrect information (some names and titles were incorrect), I find it a fair biography of Maria Theresa. He covered both MT and Frederick the Great in almost equal parts in the course of the two wars, and I must say, Frederick the Great comes off better (even if he started their animosity), and I am no fan of his (except that I will acknowledge that he was truly a genius).
The atrocity the author mainly pointed out was the Seven Years War, which was hatched by MT and Kaunitz. Why was the minister Kaunitz so highly favoured by her, for instance? Because he was the only one in the council who supported her intent to get back at Frederick the Great and recover Silesia. Kaunitz could boast of no special diplomatic success prior to being in charge of the Foreign Ministry and being Chancellor. In addition, he was a libertine who was a disciple of the Enlightenment, he was absurd in his eccentricities, etc. In short, not someone who would likely have MT's confidence... In Kaunitz, she found someone who shaped her grudges and resentments into a new policy, i.e. the alliance reversal in favour of France.
There were many accounts in the book that clearly showed MT's need for revenge and/or feed on her grudges aside from Frederick the Great, with the Hungarians and Jewish population of Prague for example. Even her ministers had to plead with her that her orders were inhumane (i.e. she heard unsubstantiated gossip that the Jews in Prague profited from her war with the Bavarian Elector, Emperor Charles VII, and since she hated the Jews anyway, she gave orders that they be banished to nowhere and must leave Prague in the middle of the winter). Accounts that are easily verified by history....
Overall, I think the book, while it had some inaccuracies and fanciful accounts, still flattered Maria Theresa. At the end, it even presented her as very wise and "mellow"(?). But it presented her errors and motives in detail that supports the author's claim. I can post some excerpts/examples here. The author also presented many of MT's good actions and decisions, i.e. wanting to have better conditions for the serfs in Hungary and trying to industrialise that country (which the Hungarians stubbornly resisted).
And yes, her relationship with Franz Stephan was unsatisfactory on both sides - but although Franz Stephan has been portrayed as the 'softer' partner, and credited with a more amicable relationship with his children, he was fully as determined as she to force their son Joseph to marry again after Isabel's death, which Derek Beales' biography clearly demonstrates. Yet it has been Maria Theresa who was usually given the complete responsibility for this parental insensitivity, which was actually not the case. I agree absolutely with the view that Maria Theresa was not as great as she's often depicted, especially in her family relationships.
I agree that Franz Stephan in Joseph's case (and also with Mimi, had he lived longer, she would've likely been married to her cousin the Duke of Chablais instead of Albert of Saxony) was also insensitive. I also doubt if FS had much time for his children, especially the younger ones, as he was credited with. But I give him credit for wanting to keep peace with his wife, at least in public, despite his strong feelings against some of her decisions and after making the (extremely painful) decision of renouncing Lorraine in order to marry her, he never seemed to bring up said matter again, no matter their differences.