hmm, in the books it was kind of from a 3rd person perspective and the protag in this wasn't portrayed as the villain. the MC was just portrayed as one of the villains and the protag was a normal disciple? it's just like reading a book about a young hero and his journey to immortality and getting a harem and vanquishin villains. and MC was one of the starter villains portrayed in the book (it didn't descirbe MC's feelings or so, it was all from the protag's perspective)
the point is that in the book, the story was about the protag, what really happened and how the other people felt wasn't described. like the accident: the protag didn't know he was the cause of the brother's death, he also believed that MC was a villain, etc.
BUT that's not the real point here anyway: now, when MC reincarnated, someone from the modern world transmigrated into the protag's body. so this person is now the protag and he regards this world as a fantasy book that he really liked and looks forward to all the happenings bc in the end, he will have a harem. this transmigrated protag's character is quite scummy and villainous, bc in the first chapter he was willing to sacrifice his childhood friend just to get the plot developing further. the scum protag's goal is to reduplicate certain happenings so he can get his happy end with his harem.
MC doesn't know that the original protag was changed, btw.
...hmm, it sounds quite confusing. but the main reason for the appearance of the book is that MC can look and know how it really happened from another prespective, because he didn't know the cause for all this hate from the sect people in his first life.