Rejecting the prosecutor's request for convictions on murder and attempted-murder charges, jurors in Torran Lee Meier's murder trial found him guilty of three manslaughter-related counts for attacks on his mother and half-brother. Calling the Canoga Park teen-ager's life a psychological nightmare, jurors said they believed that Meier, 17, was under extreme mental duress, after a lifetime of abuse by his mother, who constantly berated him and was sexually provocative in his presence. They said the youth acted in a fit of rage, rather than in a deliberate manner when he strangled his mother, Shirley A. Rizk, 34, put her body in a car, set the vehicle afire, then pushed it over a cliff in Malibu, with his half-brother Rory Rizk, 8, also inside. The youngster survived. Describing the torment Meier experienced almost since birth, one juror said, "We were surprised he didn't commit suicide." Meier faces a maximum sentence of 13 years in prison.