The American

Jack (George Clooney), a gunsmith and contract killer, and his lover, Ingrid (Irina BjÃ¶rklund), are relaxing in Sweden. Jack becomes alarmed by a trail of footprints in the snow and pulls Ingrid towards shelter. Sniper gunshots ring out. Ingrid sees Jack pull a gun from his pocket and shoot the approaching sniper. Knowing his identity is in jeopardy, and with little hesitation, Jack shoots and kills Ingrid. He flees to Rome and contacts his handler Pavel (Johan Leysen), who insists that Jack cannot stay in Rome. Pavel sends him to Castelvecchio (a small town in the mountains of Abruzzo). Jack becomes nervous, and, disposing of the cell phone Pavel gave him, goes to nearby Castel del Monte, Abruzzo, instead.
While in Abruzzo, Jack contacts Pavel, who sets him up with another job. He meets Mathilde (Thekla Reuten), who wants him to build a custom sniper rifle for an assassination. He also begins patronizing a prostitute, Clara (Violante Placido), and they begin a relationship separate from her business duties. Jack meets with Mathilde to test the weapon. She is impressed by the craftsmanship, but asks him to make a few more adjustments. Later, Jack realizes that he is being followed by an assassin from Sweden, whom he kills.
Jack is tormented by dreams of the events in Sweden and regrets killing Ingrid. His friendship and conversations with a local priest, Father Benedetto (Paolo Bonacelli), encourage him to question his life choices. Both Mathilde and Clara notice Jack's association with butterflies, Mathilde by his expertise in endangered butterflies and Clara by his prominent tattoo. When Father Benedetto tells Jack he senses he lives in a special kind of hell, "a place without love," Jack starts to let himself feel love for Clara, and envisions a life with her.
Jack talks to Pavel one last timeâasking how the Swedes have found him. In his growing fear, he even suspects Clara when he discovers a small pistol in her purse. He questions Clara about the gun and is satisfied with her explanation (to defend herself, since there have been newspaper reports of murdered prostitutes).
Jack agrees to deliver the weapon to Mathilde as his last jobâbut at the last moment he opens the briefcase and sabotages the rifle. At the drop-off, Jack becomes suspicious that Mathilde plans to kill him. Before anything can happen, a busload of school children arrives. Mathilde gives Jack his paymentâa thick envelope full of cashâand the two separate. As Mathilde drives away, Pavel contacts her and asks if she has killed Jack. She tells him she has not, but says she is following him and will kill him.
Clara meets Jack at a religious procession in town. Jack asks her to go away with him and she agrees. Mathilde tries to shoot Jack from a nearby rooftop, but the sabotaged rifle explodes in her face. Seeing Mathilde fall from the roof, Jack gives Clara the envelope full of cash and tells her to go to a river where they had picnicked and wait for him. He runs to Mathilde, who is dying on the pavement, and discovers that she also works for Pavel.
As Jack goes to meet Clara, he hears Pavel behind him. They exchange gunfire and Pavel drops dead. As Jack drives to meet Clara at the river, he feels his abdomen and realizes he has been shot. Jack arrives at the picnic spot and as he sees Clara, he collapses. Clara screams and runs to the car. A white butterfly flits skyward from Jack's car.

Question: What does Jack regret?
Answer: Killling Ingrid.