impressive We all know the solution to this specific Poirot mystery: 'they all did it'. So when working my way through this series i wasn't even sure it'd bother to watch this episode.
But what a pleasant surprise: the writers also realized that the viewers know the basics so they just go through the motions in that regard (the kidnapped baby, the letter 'H', the false evidence, etc). Far easier than in any other Poirot-episode do we get to the truth: they all did it, and they aren't ashamed, in fact the killers are practically bragging about their clever ruse (which makes sense, i suppose, of course they are happy to let everyone know about their revenge).
Where most episodes are a 'who done it' this episode is all about Poirot's moral choice: this is a man whose 'holy mission' is to expose the truth and let the law take it from there. That truth may come at a high cost, but he will not be guilt-tripped for exposing it: if 'a good man' lied and committed suicide than that's his fault/choice and not Poirot's.
The intro with the stoned woman in Istanbul doesn't make much sense in other versions of this story, but here it all kinda comes together. Poirot respecting 'the law of the land' and keeping his opinion to himself (keep in mind he is _not_ English). His faith that he is an instrument of god. The struggle it is for him to turn a blind eye.
An element i haven't seen mentioned in other reviews is the racist attitude of the killers (if there is anything 'typically English' to be found in this tale it's here, in the nonchalant prejudices). Of course every judge in the world would go light on them, would bent over backwards to let them get of with a slap on the wrist. But that's not what they are after, their fear is not as much facing a judge but facing a Yugoslavian judge. To them the entirety of eastern Europe is fly-over country, a backwards backwater with less justice than the corrupt Chicago judge who set the child-murdering mafia-guy free. That's a hard sell for a war-refugee: to say 'we do not acknowledge local legal sovereignty'.
Poirot was thus far presented as 'the perfect detached gentleman': a nice guy, who is always emotionally-stable and whose emotions never appear to run very deep. Here we see a different view of him, a view that is probably shocking to those who equate 'intelligence' and 'intelectual rigor' with 'millitant atheism'.