Could Freud, the celebrated psychoanalyst, and doctor, have been a serial killer? And the obvious answer to this bizarre question is, of course, no. Anyone, but not Freud. But why not? In fact, there have been many serial killers among doctors.
As Freud’s contemporary, Conan Doyle, a doctor himself, insightfully observed, When a doctor goes wrong, he is the first of criminals. He has nerve and he has the knowledge.  And in this case, we have to do not with one, but with two murderous doctors, Freud and Fliess, killing in tandem. (1)
What also makes us doubt that Freud could have been a serial killer is the fact that, at least in his later age photos, he appears so normal. But so do most serial killers.
They are just like us, except that they kill for enjoyment. Serial killers, go about their business and ordinary lives just like the rest of us. ... they don’t look or act like monsters, and that is why they become successful. We see them, but we look right through them. What makes them monsters is not how they look; it’s that they “don’t lose any sleep” over what they do. (2)

Bizarre, complex and devious

Freud was one of those monsters hiding in the plain view. As one of the most capable Freud researchers, Peter Swales explained, Freud was, an exceedingly bizarre, complex, and devious individual - indeed a man of such masterful artifice that probably to most mortals he will forever remain inscrutable. (3)
And in his Reflections on War and Death (1918), aged 62, reflecting his own mentality, Freud claimed that the mankind, Like primeval men, we are a gang of murderers, (4) which is obviously not true. After all, unless Freud is concerned, murderers are an exception rather than the rule.

Incest, cannibalism, and lust for killing

In The Future of an Illusion (1927), aged 71, describing his own urges while ascribing them to the rest of humanity, Freud explained that, Among these instinctual wishes are those of incest, cannibalism, and lust for killing. (5) It cannot be stated any clearer. And the man practised what he preached.

Freud's mask

How was it possible that the truth about Freud’s true nature had remained such a well-preserved secret for so long? The German writer, Stefan Zweig, who knew Freud personally, provided the following explanation, Every genius, says Nietzsche, wears a mask. Freud has chosen one of the most opaque of these, the mask of inconspicuousness. … of Mr. Everyman; but behind the mask … a demon, is at work. (6)
In other words, Freud was the personification of Dr Jekyll and Mr Hyde, a doctor on the surface, a dangerous criminal underneath. And maybe Freud was worse than a demon.
Did no one, during Freud's lifetime, have an inkling about Freud's murderous deeds? Apparently, someone did. Even though, In his biography of Freud, one of Freud’s Viennese disciples, Sachs, stated that he had, no revelations to make of secret misdeeds. he was happy to provide a clue in this regard. Thus, referring to his relation to Freud, while comparing himself to Boswell, the biographer of the writer, Samuel Johnson, Sachs pointed out that, Boswell did not bring to light that Dr. Johnson had committed murder or even adultery. (7) And, if we replace Boswell with Sachs, and Dr. Johnson with Dr. Freud, the allusion becomes obvious.
Thus, Sachs stated that he did not bring to light that, Dr. Freud had committed murder or even adultery. Apparently, in this surreptitious way, Sachs wanted to convey an indication about the criminal nature of Freud's darkest secrets. That Freud committed adultery - nothing much to blame him for - had been a question of open discussion, while, on the other hand, there had been a remarkable silence, except for a couple of writers, about Freud being a murderer, a serial killer even.
(1) Doyle, Conan, Adventures of Sherlock Holmes: The Adventure Of The Speckled Band, (1892, p. 198).
(2() Douglas, John & Olshaker, Mark, Obsession, (1998, p. 9).
(3) Malcolm, Janet, in the Freud Archives, (1984, pp. 133-135).
(4) SE 14, p. 297.
(5)  SE 21, p. 10.
(6) Zweig, Stefan, Mental healers: Franz Anton Mesmer, Mary Baker Eddy, Sigmund Freud, (1962, pp. 276-277).
(7) Sachs, Hanns, Freud, Master And Friend, (1944, pp. 9-10).

Writing to Fliess on April 6, 1897, Freud conveyed an odd message, I would gladly do without all these complications, but, as you know, Que messieurs les assassins commencent.  [Let the murderers begin.]
And he explained what more in detail what he meant, It is like the series [of periods]; if there were only two, they would be accepted more readily. *
Now, what does this statement mean? Let's disregard the editor's addition of “periods”, as an explanation of the “series”, since it doesn't occur in the original passage, and only confuses the reader about the true meaning of the statement. Without doubt, as it his wont, the strange Freudian statement contains a secret message, that was only intended for Wilhelm.

A series of murders

This is Freud's original message, I would gladly do without all these complications, but, as you know, Que messieurs les assassins commencent  [Let the murderers begin]. It is like the series; if there were only two, they would be accepted more readily.
Let's analyse the second part of the strange statement that said, It is like the series; if there were only two, they would be accepted more readily. A series of what? Two of what? What is it that is committed in a series? So, two would be more acceptable, but more than two, thus the series would not be accepted as readily?
Now, taking Freud’s proclaimed penchant for murder, the meaning of the statement becomes clear. Two murders are not as suspicious as a series of murders. Killing more than two, and the authorities could suspect a series of murders, and put more energy into finding the culprit or, as in this case, the culprits.

Only two murders

Actually, Freud's statement appears incomplete. How about if we add murder to it? Then what we get is the following, It is like the series of murders; if there were only two murders, they would be accepted more readily.
Or rather, two murders would not have been as suspicious as a series of murders. Voilà! Thus, the message to Fliess states that they should, at least on this occasion, limit the number of killings to only two. Not bad advice at all, since it had worked for over a century keeping the world oblivious to the fact that Freud was one of the most successful, and prolific, serial murders of our times, killing for over half a century.
* Freud, Sigmund, The Complete Letters of Sigmund Freud to Wilhelm Fliess, 1887-1904, (1985, p. 235).

What confirms the conclusion that the series refers to murders is Freud's quote in French that states, "Que messieurs les assassins commencent", thus in translation, "Let the gentlemen murderers begin". Who were those gentlemen assassins? And the answer is obvious, it was Freud and his likewise murderous lover, Fliess.
Now Freud's statement, I would gladly do without all these complications, apparently referring to Fliess’ preceding letter regarding earlier committed murders - arguably more than two, and the following investigation - makes sense.
Where the new murders were planned to be executed? Definitely, not in Freud’s native Vienna, and not in Fliess native Berlin.  The two assassins were too clever for that. Killing close to home was never a good idea. And, at the time, the police had no way of tracing a murderer, who only came for a guest "performance", and thereafter left for home.

Street walking in Venice

As Freud had explained, he would, prefer Nuremberg to Prague, and would prefer even more to associate Venice with your periods if Venice were a city where one could take walks. In order to make sense from this passage, we can safely can ignore Freud's "periods" (after all, as men, neither of them was menstruating).
Keeping in mind that we have to do with two serial killers looking for their prey, let's rewrite this passage, so that it makes more sense. And if so, then it would look like that: Freud would,prefer Nuremberg to Prague, and would prefer even more ... Venice ... where one could take walks.
Apparently, generously, Freud was offering Fliess a choice of several hunting grounds. Actually, Freud's conditional statement, if Venice were a city where one could take walks, is an obvious clue. Since one can take walks in Venice, just like any other city, the statement cannot be taken at face value, but needs to be interpreted. What kind of walks would two foreign murderers be looking for? The only reasonable explanation is that they would be looking for their victims in the streets, thus streetwalkers, the easiest prey for the two murderers, on a guest appearance, in Italy.

Can't wait

Even though Venice was Freud's clear preference for the place for the killings, Freud was flexible, happy to accept alternative locations, be it in Germany or Czechia, Freud also provides the timing of their murderous trip stating, I can leave Friday evening and stay away until Wednesday morning at the latest. Let's assume - although, we can't know that - that by the return post Fliess confirmed that he accepted Freud's initial proposition of making a murderous visit to Venice.
Thus, since the letter was written (and dispatched?) on Tuesday, April 6, 1897, it is reasonable to assume that Fliess date with Freud would take place on the coming weekend. And since night travel, by train from Vienna to Venice, takes approximately ten hours, Freud would arrive to Venice, where he would meet Fliess, early on the Saturday morning of April 10, 1897. Thus, Freud would have had four days for the "walks" with Fliess, returning home on Wednesday morning of April 14, 1897. More than enough time for a couple of killings.

Actually, it appears that Fliess cancelled the murderous excursion, since Freud's next letter, again referring to the planned trip, was dated Monday, April 12, 1897. (So, they didn't meet on this weekend). But Freud was still very eager for the two assassins to meet, writing that he would, await your travel instructions, even if they should be conveyed by telegram, adding wistfully I long for a few beautiful days. (1) As a matter of course, what is a beautiful day to an assassin is not exactly the same as what is beautiful for a more peaceful person.
Since his letters are missing, we don't know what Fliess wrote in reply, but it seems that their date was not happening as yet. Thus, again, on Wednesday, April 28, 1897, Freud reminded Fliess about their, talks about [future] travel", incoherently mentioning (his dream about?), Salerno: Neapolitan - Sicilian, adding, Behind it your promise of a congress on Italian soil.