A Bay Shore teen accused of fatally stabbing his younger brother was arraigned Saturday on a charge of second-degree murder.

Police say Robert Rivera, 19, stabbed his 17-year-old brother Brian Castillo in the neck while Castillo was playing video games. It happened just after 7 p.m. Saturday in a bedroom of a home on North Thompson Street.

Officials say the victim ran out of the house, saw his father outside, and said, “Look what Robert did to me." The assistant district attorney says Castillo then collapsed on the lawn and died.

Prosecutors say after Rivera took off on foot after the stabbing. He was found by police about 20 minutes later, and was arrested.

Prosecutors say Rivera admitted to stabbing his brother and gave a full, videotaped confession.

But Rivera's lawyer denies the charges and asked the court to give Rivera a psychiatric evaluation.

There is no word yet on a possible motive, but on his way to court Rivera told News 12 his brother threatened him.

Rivera is expected to be back in court Aug. 30, once the results of his psychiatric evaluation come back.