A boatload of oddness
That old distinction between first-person shall/will and second/third-person use apparently also applies to should/would - though I don't know if it applied in the "moral obligation" sense of should. But that's not the odd thing about this quote, from a 1922 novel by Joseph S. Fletcher called The Middle of Things:
"Robbery wasn't the motive. Murder was the thing in view! And why? It may have been revenge. It may have been that Ashton had to be gotten out of the way. And I shouldn't wonder a bit if that wasn't at the bottom of it, which is at the top and bottom of pretty nearly everything!"And it's not the multiple negations (shouldn't be surprised if that wasn't to mean "wouldn't be surprised if it was" or "would be surprised if it wasn't") - that's expletive negation and it's very common in speech (if deprecated in formal writing) (see this Language Log post for more on the construction). After all, this is dialog in fiction; it's perfectly acceptable.
"And that, ma'am?" asked Mr. Pawle, who evidently admired Miss Penkridge's shrewd observations, "that is what, now?"
No - it's the use of that. The first time I read the clause "if that wasn't at the bottom of it" I thought "that" was referring to "getting Ashton out of the way", and I couldn't figure out how getting him out of the way was "at the top and bottom of pretty nearly everything."
But Miss Penkridge is using that cataphorically - that is, to refer to something that hasn't been said yet - and then the referent is another pronoun (which). This sort of splitting of the "that which" reads very oddly to me. It's as if she's dropping out a noun phrase and just keeping the two relatives ("the thing that was at the bottom which is the thing at the top and bottom...").
The sentence isn't by any means impossible to parse, though I couldn't without the clue provided by Mr. Pawle's follow-up question (that is what?) - but I don't think I've encountered it before, certainly not very often. (It's not helped by the misplaced comma before which, either; surely that's a restrictive relative clause, defining which "that" she means.)
Nowadays, I think you'd be more likely to see a pseudo-cleft structure instead, something like this:
And I shouldn't wonder a bit if what was at the bottom of it is what is at the top and bottom of pretty nearly everything!And if nothing else, we'd understand what she meant as easily as Mr. Pawle did the original.