Garcin's cowardice. Therefore, Garcin's particular hell is having to be confronted eternally by a woman who relentlessly judges him and reminds him without respite of his cowardice.
Estelle's sexual attractiveness to Garcin offers him the possibility of momentary release from his suffering, and she desires the same release, but when they try to fulfill their desire, they are unable to do so because of the taunting of Inez. Hell for Garcin, then, is having to be conscious eternally of his cowardice, trying eternally to get Inez to share his rationalization, and being eternally rebuffed.
The first sign of Garcin's attempt to rationalize away the reality of his situation comes when he suggests that the three being put together in hell is a "fluke." When Inez laughs at this notion, and Garcin asks her why she is laughing, Inez says, "Because you amuse me, with your 'flukes.' As if they left anything to chance! But I suppose you've got to reassure yourself somehow" (14). In fact, the effort to reassure himself---through the winning of Inez's approval---is what will occupy Garcin's time for all eternity.
On the other hand, Estelle is willing to accept at face value whatever Garcin says about himself, and when he changes his story, she accepts the new version of his life as quickly. When she finds that he is a coward, it means little to her. Like Garcin, all she wants is comfort, escape from the reality of her life (she murdered her child), and she is more than willing to enter into a mutually-forgiving-and-rationalizing pact with Garcin in order to win that comfort and escape. When Garcin offers his first story that he was a pacifist and shot because he stuck my his principles, Estelle immediately starts to call him a hero, but she is interrupted by Inez who speaks the word "ironically"
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