In some reddit circles people suggest that Sebag-Montefiore is overly-relied on to flesh out the theory, and that it's part of an anti-communist project.
Can someone answer the question in such a way so as to maximally address such concerns?
My intuition is that the rejection of the allegation "tankie bs" but I haven't been able to build a case.
It is actually something that has merit, and not just from Sebag-Montefiore's writings. Amy Knight, who wrote the only (as far as I know) English-language biography of Beria, Beria: Stalin's First Lieutenant (1993) notes that he had a voracious appetite for women, and it was one of the accusations his opponents brought against him when he was arrested (they had a list of 39 names). While the latter obviously was politically motivated, it seems that there is some basis to the stories. I don't have the book on me at the moment so can't get the specific details she mentions, but can do so (for tomorrow), if needed.
I'm a bit fuzzy on the finer points, as I haven't kept up on my reading, but I believe The Sword and the Shield (a book about the Mitrohkin archive) said that the KGB archives had detailed that his arrest and execution were due to being found to be a pretty monstrous sexual criminal, and there may have been murder to boot (again, a while since I read it, grains of salt) and that the trial for crimes against the Russian state and the Revolution was simply a show trial to cover the reality that couldn't be said, that someone who was (I believe) set to take over the Soviet intelligent apparatus was actually the sort of monster that the soviets believed could only be the product of western society and it's problems.