Schwarzer's thesis is that Kelly was killed by Bastian, who wanted to escape from her suffocating embrace. He then shot himself. Kelly apparently used to tell Bastian, with startling frequency: 'I cannot live without you.' His logic, says Schwarzer, was that if he could not leave her when she was alive, he would have to leave her dead.
The deaths, in October last year, caused shock throughout Germany. Kelly and Bastian were two of the most charismatic figures in German politics: the former general, who had moved from the Bundeswehr, the German army, to the peace movement, and Petra Kelly, the MP and world's most famous Green.
Within days of the discovery of the bodies in the house that Kelly shared with Bastian in Bonn, conspiracy theories abounded. The official version was 'double suicide'; later, it was acknowledged that Bastian almost certainly pulled the trigger not once, but twice. Kelly is thought to have been asleep. But the motive remained obscure.
Schwarzer is scornful of the fact that a memorial ceremony held in Bonn two weeks after the bodies were discovered was 'for killer and victim alike'. She is critical of the treatment of Bastian and Kelly as heroes in death, as well as in life.
The book portrays 44-year-old Kelly as an obsessive neurotic, who could scarcely move without the 69-year-old Bastian to attend to her every whim. She went to bed at four or five in the morning, leaving him notes of errands to carry out in the morning; she hated him to leave her even for a short while. She became ill whenever Bastian went to visit his wife, Lotte, with whom he remained on friendly terms, and of whom he used to say: 'I want to grow old with her.'
Bastian's and Kelly's life appears to have been full of unusual triangles. Palden Tavo, a Tibetan lover of Kelly's, accompanied her and Bastian on some of their trips. When Mr Tavo sought to break off the relationship, Bastian wrote letters imploring him to renew contact with her. In Schwarzer's account, Mr Tavo noted in his diary a comment from Bastian: 'If things go really badly, then I will go - and take Petra with me.' Bastian's family, on the other hand, suggest that he may have suffered a heart attack, then killed Kelly and himself.
Bastian's son, Till, has accused Schwarzer of describing conversations between himself and his father that never took place. But Bastian's widow, Lotte and daughter, Eva seem to have accepted the book to some extent.
Eva Bastian argues that her father carried on, with 'soldier's duty', even when he was 'sick and no longer really able to support her'. She said: 'I cannot forget the friend of both of them, who said to me at the memorial service in Bonn: 'We all hid behind Gert's broad shoulders'. '
There are rumours that Bastian may have been a part-time agent for the Stasi, and may therefore have been subject to blackmail. But the true reason for the deaths is unlikely ever to be known.
Kelly's friends probably expected that, in Schwarzer, a well-known feminist author, they would find a sympathetic chronicler.
In that, they have been disappointed. Three of those closest to Petra Kelly - her mother Margarete, her best friend Erika Heinz, and her close colleague Lukas Beckmann - who are thanked at the beginning of the book, have complained of the 'sheer cynicism' of Schwarzer's writing, and have demanded that the 'unasked-for' expression of thanks be removed. According to their statement, Schwarzer's book is 'journalistically and humanly impermissible . . . a misuse of the dead for her own ends, and a falsification of life'.
Beckmann describes A Deadly Love as a 'voyeuristic piece of work, sloppily researched'. He accuses Schwarzer of inventing quotations and getting what she needed by deceit.
But Schwarzer denies inventing quotations, and argues that the attacks on her are merely a way of deflecting the main arguments. 'Uncomfortable truths are to be suppressed again, this time by discrediting the author,' she said. She blames the 'bad conscience' of Kelly's and Bastian's friends towards the two victims, who were 'very lonely, by the end'.
With pious anger, Schwarzer argues: 'Two people are supposed to be turned into icons. And now the kitsch which drove them to their deaths is supposed to draw a veil over their lives and work.' The book has been serialised, and is selling well.