Roazen's book is the story, not of a suicide, not even of a manslaughter, but of a murder in which Freud is unmasked as the culprit.
Eissler, Kurt, R., Victor Tausk's Suicide, (1983, p. 3).

Who wrote the note?

There is a number of reasons why the note appears to not have been written by Tausk.
1. The note doesn’t sound like a message to one’s sworn enemy, on the contrary, the letter is both reverential and humble.
2. Would Tausk, at all, write a letter to Freud about not coming to the meeting, since it wasn’t uncommon for Freud’s disciples to be absent from the meetings?
As Roazen explained, by the beginning of 1909 the Vienna Psychoanalytic Society had only twenty-eight members, and it was unusual for more than eight or ten to come to meetings.
Apparently, the presence at the meetings was not compulsory. And It is doubtful that the members were expected to notify Freud about being absent. In fact, it would be bizarre if Freud received several absence notices, whenever the meeting was taking place.
3. Notably and falsely, the content of the note shows Freud and his relationship with Tausk in a good light. And, Tausk’s greeting, Dear professor, intimates that Tausk and Freud were on good terms, while the claim that Tausk was, solving the decisive affair of his life, no doubt, is a “hint” that he was contemplating suicide.
Furthermore, Tausk’s statement that he didn’t want to meet Freud because he didn’t want to ask for help with his decisive affair, intimates that Freud would be happy to help Tausk with his problem which is again, to say the least, improbable.
4. Bizarrely denigrating himself, the Tausk stated in the note that at the time of writing the note he suffered from a neurosis, in other words was mentally unstable, this statement again making the suicide explainable. Would Tausk denigrate himself in that way:?
5. Ending the note Tausk, allegedly, sent cordial respectful regards, gratefully greeting Freud, which, considering their animosity, again is highly unlikely.
6.  Last but not least, the fact that Tausk signed the note only with his surname is suspicious, and symbolic, since Tausk’s first name, Victor, was omitted. As it is apparent from the deadly outcome of his conflict with his former master, it was Freud, rather than Tausk, who was the victor in their conflict.
7. Notably, it was Freud's habit to sign his letters with only his surname, Freud.
8. Another question is, of course, how and why, and in whose possession, such an inconsequential note survived for half a century. In fact, it would be remarkable it was kept by Freud, since on several occasions, the last time, when going to his exile to England, destroyed much of his papers. And if it was in Freud's possession after that many years, why would he keep such a note?
9. Notably, although the note, as its dating shows, was written on the day of the meeting, apparently it arrived in time to inform Freud about Tausk’s absence. And the obvious question is whether it even possible that the note would be delivered to Freud on the same day it was written? Knowing how the post office functions, it is out of the question since there are no same day deliveries.
And if the note was delivered in person, it would exclude Tausks' involvement. What would be the point of Tausk delivering the note if he could tell Freud in person that he wouldn't be coming? Thus, unless someone else delivered Tausk's alleged absence note, the whole story about Tausk notifying Freud about his absence in writing is a ruse, and the alleged note is a forgery.
Who wrote the actual absence note, we'll never know for sure. On the other hand, everything points in Freud direction. Had Freud been the murderer, this is exactly the kind of letter that a murderer would have written to cover up his conflict with the victim and his involvement in his death.

Marriage license

On this very day, thus, on the Wednesday, July 2, 1919, the day before his alleged suicide, Tausk, was to go for the marriage licence.* 
As a matter of course, no one intending to marry the next day kills himself. Moreover, nothing in Tausk's behaviour on the day indicated his coming suicide. On the contrary, Tausk dedicated his time to everyday chores. Thus, he spent the afternoon with his seventeen-year-old son, with whom he had supper. As Tausk informed him, he was going to a concert in the evening where his fiancé was performing. And since he was going to see her, apparently, there was no change of plan, and he had already acquired the marriage license.

Don't worry about me

These are not the actions of a man going to take his life already the next morning. According to his son, Tausk was rather worried, although his last words to him were, Don’t worry about me. This doesn’t sound like the suicidal father saying farewell to his son.
That evening Tausk wrote a letter to his … favourite sister informing her of his planned wedding and thanking her for the cigarettes ... and bacon she had sent him. The letter doesn’t mention anything about Tausk contemplating suicide.
And, if the letter was written in the evening, since Tausk was found dead the next morning, how did it get into the mailbox? Did Tausk have stamps at home and posted it himself this evening? Or, did the murderer take the letter with him, and sent it the next day?
Thus, in Tausk's behaviour nothing indicates the upcoming "suicide". On the contrary, his actions show that he had planned to go on living.
1. He obtained a marriage license.
2. Was going to his fiancé's concert.
3. In a letter, he informed his sister of the upcoming wedding.
4. Thanked his sister for cigarettes and bacon!
5. Didn't say farewell, neither to his sons, not to his ex-wife, nor to his sister.
6. No one had any inkling that Tausk was  contemplating taking his own life.
In particular, mentioning mundane items like cigarettes and bacon, rather than saying farewell, shows, without doubt, that this not a message of someone planning a suicide. Rather, apparently, Tausk was looking forward to consuming more of his sister’s gifts in the future.

Chose suicide?

Bizarrely, Roazen explained that, Apparently he had not yet decided to kill himself. Apparently! Had Tausk contemplated suicide at all, at the very least, he would have, in his letter to her, said farewell to his sister. It is also inconceivable, should Tausk contemplate suicide, that neither his sons, nor his fiancé, not his ex-wife, nor any of his friends and acquaintances, had any inkling of his suicidal state of mind. The way Tausk acted, occupying himself with everyday chores, was not a behaviour of a suicidal person but of a person looking forward to the future.
Absurdly, since he knew nothing about Tausk's musings on that particular occasion, Roazen claimed that, the precipitating cause of Tausk’s suicide was certainly his inability to go through with his marriage, explaining that, for him there was no way out.**
Of course, there was a simple way out! Tausk could break off the upcoming marriage.  Instead, he obtained the marriage license, and went to see his fiance's performance. He wasn't looking for a way out!
Remarkably, Roazen came to the grotesque conclusion that for Tausk, life became more painful and tortured than death. Dying can be less threatening than living. So Tausk chose suicide. 
How wrong can you be? Why had Roazen decided to speak with two tongues now? Was it because he was fearing Anna Freud’s reprisals?
As it is apparent from his actions and words, Tausk wasn't having any thoughts about killing himself. So, how is it possible that, the very next day, he was found dead?
* Roazen, (1959, pp. 122). ff
** Roazen, (1959, pp. 120).

Tausk's son, Marius', about the death of his father

This is what he had to say: In 1908, my father came in touch with Freud, and this must have meant to him a completely new outlook on life. (1) Ironically, Tausk's encounter with Freud would mean for him - even though, unfortunately for him, he wasn't aware of it - also a completely new outlook on death.
This is what Marius had to say about his last meeting with the father in the first days of July, 1919, immediately preceding his death. As he pointed out, he had a guilty feeling for not having sensed the magnitude of the conflict that must have raged inside him. Poor Marius, he was accusing himself throughout his life of not feeling a conflict raging in his father, oddly not asking himself whether there even was a conflict that could result in his father's suicide. In fact, nothing his father did, or said, during their last meeting, indicated that there was one. Rather, the father behaved as one would expect of a "normal" person meeting his son.
This is Marius' recollection of their last meeting: On that second of July, when I had supper with my father, before he was to leave for a concert where his fiancée played the piano and I had to go to the station to catch the night train for Graz, we had some cold cuts and my father put some beer on the table. Going to see the fiancé playing, offering his son food and drink, this doesn't sound like a behaviour of a suicidal man. Moreover, Tausk was in a good mood since, as Marius recounted, He teased me because I would not touch it.

Never mind what I do

Again, nothing, in the father's behaviour, indicated that a Tausk was suicidal, on the contrary: When we parted ... he simply said: "Kümmere Dich nicht um mich." This could be translated as: "Never mind what I do." Or maybe, a better translation would be, "Don't worry about me". Just a thing one could say to one's son when one is not contemplating a suicide.
Apparently, Tausk didn't confide in Marius his problems with Freud. Maybe he didn't feel it was important enough to share it with his son. For that reason, Marius' naivety, when it comes to the relationship between his father and Freud, is understandable.
his is what Marius had to say about the issue: before I met Dr. Roazen, it never occurred to me, that this relationship could have played any role in the causation of my father's suicide. ... I had read his will and his farewell letter to Freud and I was—and still am—convinced that these documents contained nothing but the (subjective) truth. (3)
He may have been right about the documents containing the subjective truth but the question is whose truth. Was it his father's or was it Freud's. But he seemed to have accepted the idea that his father's "suicide" was the result of his conflict with Freud. When Dr. Roazen presented me with his "rejection hypothesis", Marius' wrote, I was utterly incredulous but I confess that when I read the complete story, it impressed me as far more convincing than I had anticipated. (3)
One can only agree that the idea that Tausk would've committed suicide because he wasn't going to be analysed by Freud is absurd. Ironically, Marius died believing that his father committed suicide and that Freud wasn't involved.
(1) Tausk, Marius. Marius Tausk: Victor Tausk as Seen by His Son. American Imago, vol. 30, no. 4, 1973, pp. 323–35. p. 325.
(2) Tausk, (1973, p. 332).
(3) Tausk, (1973, p. 333).

Continues with Part 3