Love and hate seem to cohabit the hitherto unpublished correspondence between one of the richest bankers in France and his Swiss mistress, who went on trial in Geneva last week accused of killing him during a bout of sadomasochistic sex.

Presented in court, the e-mails and text messages of Edouard Stern, head of a French banking family, to Cécile Brossard, his lover, suggested a deep-rooted obsession that ended when she shot him one February evening four years ago. Police found him tied to a chair and dressed in a latex suit.

“You are a sh** and I vomit on you,” writes Stern on one occasion, and on another: “I love you madly, for ever. Your little devil.” He says how “sad” he is that they “scolded” each other. Later he writes: “You’re just a whore. You would have worked for the Gestapo during the war. You would have turned me in as a Jew.” The defence has tried to present the killing as a “crime of passion” in which Brossard, a 40-year-old former escort girl and shop assistant, exploded in anger when Stern, alluding to a gift of $1m (£608,000) that he was taking back from her, said: “A million is a lot to pay for a whore.”

“At that moment there was only hatred in her,” Jacques Gasser, a psychiatrist, told the court. She took one of Stern’s pistols from a drawer and shot him first in the head, she testified, tears streaming down her cheeks, then three more times: “I was out of my mind, beside myself.”

Referring to the latex “body stocking” and mask warn by Stern, 50, she went on: “I felt like I was shooting at a plastic doll, not at a man.”

Her lawyers have said that Brossard, who had been raped at the age of 11, was tormented by Stern, a friend of Nicolas Sarkozy, the French president.

Xavier Gillet, a boyfriend with whom Brossard lived in the Swiss village of Clarens, claimed that after arguing with her, Stern would spy on the house with night vision binoculars, “that only the army is supposed to possess”. On one occasion Gillet agreed to meet him.

“I found myself in front of a little boy,” he recalled. “He told me he had blown his top, that he apologised and promised not to do it again. He was jealous, he said, because Cécile was seeing young men in Paris.”

Béatrice Stern, the banker’s widow, and their children Mathilde, 24, Louis, 22, and Henri, 18, sat with stony faces through part of the proceedings before leaving the court. They had no desire to hear some of the evidence – the voice of a man who was not the father and husband they knew.

Brossard kept telephone messages containing his pleas, threats and insults. The transcripts of hundreds of e-mails and text messages that Stern sent to Brossard between 2003 and 2005 fill 452 pages of court evidence and raise the question of when he found the time to run IRR Capital, his investment fund.

“You don’t exist, f*** you, I’m going to hurt you,” says Stern in one of the texts. “Nobody leaves Edouard Stern.”

Svetlana, a Russian woman with long blonde hair, testified that Cecile had once invited her to join the couple for a romp in a hotel room. The women were dressed as schoolgirls. After sex, Stern sat reading a magazine article about the top 100 fortunes in the world. “He was disappointed not to be mentioned in it,” she said.

In 2004 the couple flew on Stern’s private jet to Tanzania, where they went on safari. He later offered her £13,000 a month and wrote that he wanted to marry her and would give her $1m. “The most beautiful way of spending this money would be to give it back to you, to prove in turn my love for you,” she wrote to him.

He paid her the million but when she seemed to be in no hurry to give it back he froze the funds in her account.

Brossard, who is reported to have attempted suicide while in prison, claims that she still loves Stern and “his soul is in me”. The prosecution, however, has presented her as a calculating gold-digger, noting that she initially told the Geneva police she was innocent. They are demanding a harsher penalty than the maximum 10-year sentence Swiss law allows for unpremeditated killing.

For the defence, there can be no doubt that the crime was not premeditated: if Brossard had kept up a relationship with Stern for financial gain, it is argued, why would she have killed him?

Her lawyer notes that on the night of the shooting Brossard had arrived at Stern’s flat equipped for sadomasochistic sex. She was dressed as a “dominatrix” and was whipping him when Stern taunted her about being a prostitute.

“I would like to ask for forgiveness,” Brossard told the court last week, “but we cannot seek forgiveness for something so abominable. The only thing I can do is to try to explain the truth.”

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