I've read a little disappointment with the 'twist ending', but for me the twist wasn't there, but well before that point, in the revelation of the letter. Here we had the narrator, who had previously presented himself as genial and easy-going to a fault, reveal a completely different side. But it was a shock to himself, too, to recognise he was capable of - and I think this is the right word - evil.
By coincidence, I was reading Bruce Hood's The Self Illusion just after I finished this, and at the point I went back to it he was discussing self-narratives and the "totalitarian ego", which seems to be a reference to a classic study:
Psychologist Dan McAdams proposes that when it comes to making sense of our lives, we create narrative or personal myths to explain where we have come from, what we do and where we are going.16 This is the narrative arc of our lives – the background, the struggle, the climax and resolution that people readily attribute to the story of their lives. For example, some may see themselves as victims of circumstances beyond their control, reinterpreting events to fit with this perspective. Another could take the same set of circumstances and cast themselves as the resilient hero, triumphing over adversity to get where they are today...these accounts are myths because they are not grounded in reality but rather rather follow a well-worn narrative path of a protagonist character (our self) and what the world throws at them.
I guess his point, and Barnes's point too, is that there is no such thing as a reliable narrator, ever. In this Guardian review there's a reference to his memoir:
Julian Barnes admits that he and his brother disagree about many details of their childhood. His brother, a philosopher, maintains that memories are so often false that they cannot be trusted without independent verification. "I am more trusting, or self-deluding," writes Barnes, "so shall continue as if all my memories are true."
Regarding "Mary" - well, the most famous Mary of all is a virgin mother, which kinda fits, maybe?