An El Cajon man who earlier rejected a plea bargain which would have allowed his immediate freedom was convicted Monday--for the second time--of second-degree murder. Joseph John Gentile Jr., 22, said nothing and looked downcast after clerk Mary Kaneyuki read the San Diego Superior Court jury's verdict, which followed two days of deliberations. Gentile was convicted in the Aug. 5, 1981, death of his 1-year-old niece, Tiffany Wilson, who was fatally scalded in a bathtub. Gentile was convicted the first time of second-degree murder in 1982, but an appellate court overturned the verdict because a judge omitted an instruction to the jury about involuntary manslaughter. Deputy Dist. Atty. Jay Coulter said Gentile, whom he described as being mildly retarded, repeatedly rejected a plea bargain that would have allowed him to be released on his time already served since his 1981 arrest. David Semco, Gentile's attorney, said he tried to persuade him to take the offer, but Gentile refused to plead no contest to voluntary manslaughter with a sentence of four years. Gentile insisted he wanted to clear his name in a trial. Judge Norbert Ehrenfreund set sentencing for April 22, and Gentile is expected to receive the term he got once before--15-years-to-life in state prison.