It's the inexplicable nature of this swan dive that, like the odour of cheap dog-food, allows us to identify this spoor. Now whatever you think of Abi Morgan as a writer she is professional enough to avoid using inexplicable suicides as a plot device. It's obvious to anyone with craft skills that she was going for a stonking climax where our, almost but not quite working class, hero is forced to kill a man in self defence.
This would fit in with the themes of the story and with the guilt he expresses in Episode 4. It also makes some kind of narrative sense. What I suspect was that a Gibbon intervened, at a very late stage, and said that there was no way they could have the hero kill someone, not even in self defence. We know that this must have been done late because there was obviously no time to do a proper rewrite before shooting.
And so our exciting climax is marred by pointless note which serves only to make life difficult for the writer, the actors, the director and poor bloody editor who has to cut together a scene that makes no sense whatsoever.
Thus do the gibbons rain excrement upon talented craft professionals and ultimately we the audience.