Dr Shepard, a student of Golden Albert, is a very respected person. Roger? Mr. Ackroyd, he is a trustworthy friend; In Ralph? Peyton, he is a respectable elder. Such a good impression, through the doctor's story, increased a little trust. Therefore, almost no one doubted that the doctor had anything to do with Roger's death.
But since it is the first person, it will inevitably be too subjective. In Dr. Shepard's story, we saw a gentleman and a kind old man. However, in the case of Roger's murder, the doctor tried to restrain himself and let himself hide behind the scenes, so that people clearly knew his existence, but he always ignored his connection with the case intentionally or unintentionally.
This ambivalence of desperately showing his kindness and deliberately concealing his existence is precisely the typical reaction of a murderer. In his works, he praises himself everywhere and tries to hide his slightest connection with crime. However, it is this affectation that makes Poirot catch his flaw, so he can be said to be smarter than himself.
In a word, this unique narrative perspective really gives people a novel experience. People not only sincerely admire the author's kung fu, but also deeply immerse themselves in the world created by the author for us.