An Even Bigger Plot-Hole
Just watched the movie again last night after reading the original Francis Isles novel, BEFORE THE FACT. As we know, there were many changes from page to screen, some to streamline the story (which in the novel takes place over about ten years) and some to make the character of Johnnie more ambiguous (and ultimately more sympathetic) in his motives.
The big plot-hole is this: the book's Johnnie is after money. He knows that Lina will inherit a certain sum upon her father's death. In the novel, it's more than Lina's fancy that Johnnie is somehow involved not only in Beaky's death, but in her father's - it's almost a certainty. So it is not until much later in the novel that Lina's suspicions about Johnnie really take hold.
In the film, no such amount is left to Lina upon her father's death - instead, his spiteful will leaves everything to his wife, who is to continue Lina's 500 pound a year allowance. Since her mother does not die in the film, Lina is financially where she was at the beginning.
So Johnnie's only real motive for murdering Lina would be to get her out of the way so that he could marry someone who can better afford him. Johnnie really is a complete - if charming - rotter in the novel: a liar, gambler, an embezzler, a forger, an adulterer (the maid Ethel has quite a different role in the novel) and, yes, a murderer.
Cary Grant? Oh no, really. Impossible.
"In my case, self-absorption is completely justified."