Tobby Carrier told the jury panel at the Rimouski Courthouse he had been planning to kill himself and not his family.
The accused, who was 19-years-old at the time of the attack, told the court he also had memory blanks.
He described being in a state of total panic and testified he did not know how he ended up with a knife in his hand.
The dark-haired young man broke down in tears as he talked about stabbing his brother and seeing the look of terror in his eyes.
Contrary to a pathologist's report, Carrier said he only remembers stabbing his 21-year-old brother, Ismaël, in the abdomen. He said he did not remember stabbing him a second time in the heart.
During his court appearance on Wednesday, the 12 members of the jury listened to Carrier's police interrogation. He told investigators he was physically abused as a child and had dreamed of killing his family since he was 10 years old.
Carrier did not mention abuse or revenge during today's testimony.
He described leaving the house after the attack and asking friends for help ending his own life.
When his friends asked him what happened, Carrier said he took credit for trying to kill his family.
He said he thought the attack would disprove those who thought a guy like himself would not have the guts to commit such a crime.
Last week, the father of the accused, 57-year-old Nelson Carrier, described the violent confrontation he had with his son the night the attacks took place on March 31, 2009.
Earlier this month, Carrier pleaded not guilty to the premeditated murder of his brother and the attempted murder of his parents.Suggest a correction