Lucky he had managed to pull himself together in time after that business ten—no, fifteen years ago. It had been a near thing, that! He’d been going to pieces. The shock had pulled him together. He’d cut out drink altogether. By Jove, it had been a near thing, though….
While driving to the island, Dr. Armstrong reflects, vaguely, on an egregious mistake in his past. Readers later learn that he accidentally killed a woman while operating on her while drunk. He has tried to put the event behind him—he no longer drinks and his medical practice thrives—but he still thinks about what happened. However, when a recording played for the houseguests accuses him of this killing, Armstrong denies any knowledge of the event.
Dr. Armstrong, who was standing by the window, cleared his throat. He said: “You must excuse any—er—shortcomings this morning. Rogers had had to do the best he can for breakfast single-handed. Mrs. Rogers has—er—not been able to carry on this morning…. Let us start our breakfast. The eggs will be cold. Afterwards, there are several matters I want to discuss with you all.”
Knowing Mrs. Rogers died in the night, Dr. Armstrong attempts to keep the other guests in the dark until after breakfast. As a doctor, Armstrong often appears to possess special knowledge and powers. For example, he has the power to decide what others learn and when, even about their own lives. Armstrong instinctually attempts to minimize others’ discomfort, an act born from both professionalism and arrogance
What risk was there? He’s the only person here with medical knowledge. He can swear the body’s been dead at least an hour and who’s to contradict him?
Vera explains to Lombard why she suspects Dr. Armstrong to be the murderer. Two of the murders so far involved poison. Armstrong has the power to assert medical facts without contradiction. Although Armstrong has not murdered anyone on the island, Vera’s suspicion provides a clue to how the murders are achieved: The real murderer uses Armstrong’s medical authority to trick the other guests.
Armstrong was in a pitiable condition of nerves. He twitched and his hands shook. He lighted cigarette after cigarette and stubbed them out almost immediately. The forced inaction of their position seemed to gall him more than the others. Every now and then he broke out into a torrent of nervous speech; “We—we shouldn’t just sit here doing nothing! There must be something—surely, surely there is something that we can do?”
The narrator explains Dr. Armstrong’s anxiety as the trap seems to close around the houseguests. A storm has confined him and the other four remaining houseguests in the house. In this scene, Armstrong experiences a stronger physical reaction to their enforced idleness than the rest of the group. As a doctor, he is used to acting, immediately, to save others. Sitting around waiting for something to happen goes against both his personality and training. He smokes to keep his hands busy, but the nicotine appears to only make him more jumpy.
According to my plan I should shortly need an ally. I selected Dr. Armstrong for that part. He was a gullible sort of man, he knew me by sight and reputation and it was inconceivable to him that a man of my standing should actually be a murderer! All his suspicions were directed against Lombard and I pretended to concur in these. I hinted to him that I had a scheme by which it might be possible to trap the murderer into incriminating himself.
In his note in the bottle, Wargrave reveals how he tricked the houseguests into believing he was dead: His strategy required him to appear to die, and he convinced Armstrong to pronounce him shot and killed. Armstrong’s belief in the proprieties of society made him unable to see that a man of high social standing might be a murderer. His suspicion of Lombard, the obvious choice, also reveals his lack of imagination. Wargrave may have included Lombard, in part, because he knew the unimaginative Armstrong would suspect him.