Dr. Armstrong is easily emotionally vulnerable and offended, becoming angry about the impoliteness of Tony in his reckless driving, which almost forced him off the road; when he refuses to apologize for his irresponsible behavior (which is the “correct” social response”), Armstrong comes unraveled at him. He pays close attention to the people around him and how they are affecting him, or whether they respect him or not; he doesn’t like to be thought ill of, and takes steps to apologize to Judge Wargrave for “calling out his illness like that.” He does most of his thinking with his feelings—his dislike for Vera because early on she offended him causes his bias toward her, and then he starts looking for reasons to turn around the blame on her and accuse her of being behind everything (she figures out connections too fast, and he doesn’t trust her relationship with Lombard). He points out to Blore that she and Lombard have “something going on between them,” though he doesn’t know what. He was a successful surgeon until a botched operation left him scandalized and out of favor. He is also not an apt judge of character, since his bias against Vera causes him to accuse her when she is not responsible for the murders; he misjudges every situation, refusing to take it seriously until he has actual evidence of foul play, and doesn’t believe a man is hiding on the island at first. Then he starts accusing everyone, based on different pieces of evidence, and winds up trusting the wrong person, which makes it “easy” to dispatch him.

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Armstrong is nervous, reactive, and prone to hysterics; he doesn’t trust anyone and is accusatory, paranoid, and insecure. Whenever anyone pushes him or implies he might be involved in anything criminal, he becomes defensive and guards his reputation—but the murderer also says he was “keen” to form an alliance and trusting, hoping that his usefulness would “save his life.” Armstrong first tried to create an alliance with Blore, with whom he shared some of his suspicions and paranoia, by accusing Lombard of the crimes. Driven to the edge, he indulges his 7 wing in a “careless” attitude in his final few hours—drinking, partying, snorting drugs, dancing, and having a good time, in an attempt to forget his impending death sentence.