An inmate who told police he beat his 76-year-old cellmate to death at Oahu Community Correctional Center because he was angry at an inmate in another cell was found guilty Tuesday of manslaughter.

Joseph Tui Jr. could have been found guilty of murder, but Circuit Judge Richard Perkins found him guilty of the lesser charge.

Tui, 35, faces a 20-year prison term for manslaughter at sentencing in June. If he had been found guilty of murder, he would have faced a mandatory term of life in prison without the possibility of parole, the state’s harshest penalty, because he was incarcerated at the time of the killing.

Tui waived his right to have his case heard by a jury and instead let a judge decide his fate. His trial started earlier this month. The only witnesses to testify during the case were mental health experts because Tui had also agreed to let the judge accept as fact all of the statements contained in the police reports.

Tui was asking the judge to find him not guilty by reason of insanity.

Two of the three court-appointed mental health experts and the mental health expert for the defense said Tui suffered from schizoaffective disorder, a mental illness. The third court- appointed mental health expert said Tui suffered from an antisocial personality disorder and faked his mental illness.

All three court-appointed examiners said Tui should be held responsible for his actions because he was able to distinguish right from wrong and follow the law when he fatally beat Cyrl Chung to death at OCCC in March 2013. The defense’s expert said Tui should be found not guilty by reason of insanity.

Perkins determined that Tui was not insane when he killed Chung, but he said the state did not prove murder because it did not prove that Tui intended to kill Chung or knew that beating Chung would cause his death.

Nobody saw Tui beat Chung or knows how many times Tui punched and kicked Chung. It was Tui who alerted guards that Chung needed assistance. When he admitted to police that he beat Chung, he said he had no hard feelings against him and described the beating as an assault.

Perkins also found Tui guilty of assault for punching a male nurse at the Hawaii State Hospital three months before he beat Chung.