The Supreme Court building in southern Seoul. (Newsis) South Korea's Supreme Court has upheld a 15-year prison sentence for a man convicted of beating his 87-year-old father to death after becoming angry over what he described as years of harmful treatment.The court recently dismissed the defendant’s appeal and affirmed the lower court ruling, legal sources said Friday.The man was indicted on charges of killing a relative after attacking his father’s head and face at their home in Seongnam, Gyeonggi Province, in August last year.Investigators found that the defendant had been caring for his father alone. The father had difficulty communicating due to dementia and hearing loss.The defendant reportedly flew into a sudden rage after feeling that his father had treated him hurtfully.The district court sentenced him to 20 years in prison, saying the crime carried particularly grave responsibility because the victim was his direct relative and the attack was brutal.“The victim must have suffered extreme pain before dying after being subjected to a completely unexpected attack by his own son,” the court said.An appeals court later reduced the sentence to 15 years, taking into account the defendant’s history of psychiatric treatment for severe depression and the physical and emotional strain of caring for his elderly father over an extended period.The appellate court said the crime appeared to have been committed somewhat impulsively while the defendant was exhausted by the burden of long-term caregiving.The defendant appealed, but the Supreme Court found no legal error in the lower court’s ruling and upheld the sentence.