Nicolas Holzer sat still most of the day as his trial unfolded in Santa Barbara Superior Court on Wednesday.
Holzer is accused of killing his parents, his two boys and his dog four years ago at a Goleta home in 2014.
All of the victims were stabbed multiple times.
Holzer was at the scene when the deputies arrived. and he pleaded not guilty in the case by reason of insanity.
The trial is without a jury at the request of the Public Defender.
Judge Brian Hill will determine the outcome, and if Holzer is found guilty, there will be a penalty phase to determine if he was insane at the time.