From the book and then there were none
Wargrave is a great manipulator. Really, besides Wargrave, the last four characters left on the island were Vera, Armstrong, Blore, and Lombard. Armstrong is firmly entrenched in his view of class system. He can't fathom a gentleman as distinguished as Wargrave being a murderer. His weakness is thinking that class and breeding define's a man's character. Armstrongs trust in the man leads to his own long fall off a cliff. Blore is that stupid guy who goes outside alone in one of those cheesy horror movies. Vera is tormented by her own hysterical emotions. Wargrave exploits this to his advantage. All Wargrave has to do is set up a noose for the already hysterical Vera to willingly end her life. Lombard can't believe a woman can be dangerous, yes it's sexism at it's best. Wargrave knows this so Lombard never suspects it when Vera steals his gun and shoots him.
