I believe that what Charles Darwin meant when he used the phrase “it is like confessing a murder” was that it was like letting out a dark secret. Charles Darwin was well aware of the fact that most people at that time assumed species to be unchanging as a fundamental fact of nature. For him to suggest that a species could evolve would be a complete contradiction of something that had been taken for granted as a fact of nature. Darwin knew he faced possible ridicule and scientific rejection for these views thus making it a difficult subject for him to converse with most people about. On the other hand, consider a person who has confessed to committing a murder. I think it is fairly easy to imagine a scenario in which this person now has no contact with former friends and family because of their crime. I think this is how Darwin felt with his views on evolution; if he was to suddenly begin disclosing his views on the evolution of species, then he would certainly dismissed by his friends and colleagues.
This letter was written about fifteen years prior to the publication of Origin of Species. I believe this is also part of the reason as to why he expressed himself in that manner. Since there are still fifteen years until publication, this indicates that Darwin himself had not been convinced of his view until fairly recently thus making him very cautious in admitting these to other people. Such an extreme exaggeration may also let his correspondent know that Darwin is not taking his own views lightly and has given it much thought before reaching those conclusions.
Although Robert Chambers was thinking similarly to Darwin about evolution and anonymously authored Vestiges, I do not think Chambers would have expressed in the same way. Chambers was much more concerned with the direct religious implications of evolution than Darwin was. Chambers’ main argument was that god was not actively involved in the evolutionary process. He viewed the world in more of the sense that god supplied the world with material for life to form and evolve on its own. Because of this, I have doubts that Chambers would have reflected on his views in the same way that Darwin did.