Josef Fritzl planned to kill his daughter and their incest children in acid so he could “rest in his old age”, a former cellmate has revealed.Patrick Jakob, who shared a cell in St. Pölten prison with Fritzl before the 75-year-old was transferred to Stein jail in Krems, said today (Fri) that Fritzl had revealed to him that he considered murdering his cellar dungeon family in an acid bath.”Fritzl told me he was tired of living two lives and wanted to rest. He said he had two choices – to let them all go and face justice, or kill them,” the 19-year-old who was jailed for arson told British newspaper The Sun.Jakob, now an electrical engineering student, also revealed Fritzl got “dozens of letters”. He said: “The fans saw him as the chief monster. They respected him.”The young man, however, also stressed he was surprised by how friendly his cellmate was most of the time. “You would never know what he had done,” he claimed speaking to the daily newspaper.The interview comes only days after Markus Sonnleitner, a bankruptcy trustee managing Fritzls estate, said he expected the former Fritzl family home to be demolished since liquidator Walter Anzböck could not find an investor.Fritzl was arrested in 2008 for keeping his daughter Elisabeth, 43, as a personal sex slave in a self-made dungeon basement in the Lower Austrian town of Amstetten. He was sentenced to life in prison last year.