The fact that His Bloody Project is (mostly) fictional is either the most or least important thing you could know about it, depending on how much you care.
Framed as a memoir written by an accused murderer, coupled with court transcripts and another associated documents related to the crime, HBP—down to its faux-bloody-fingerprinted cover—wants very badly to sell itself as a true-crime adventure, an In Cold Blood for fans of 1800s homicides committed in remote and sparsely populated Scottish enclaves. That the book is in fact a epistolary novel marauding as truth makes it all the more ambitious and, like its main character, all the more tricky to pin down.
Said main character is Roddy Macrae, who opens his lawyer-prescribed written statement, i.e. the book, with a fairly direct confession:
“My life has been short and of little consequences, and I have no wish to absolve myself of responsibility for the deeds which I have lately committed. [My advocate] has instructed me to set out, with as much clarity as possible, the circumstances surrounding the murder of Lachlan Mackenzie and the others, and this I will do to the best of my ability….I shall begin by saying that I carried out these acts with the sole purpose of delivering my father from the tribulations he has lately suffered.”