Joshua Komisarjevsky, 31, faces a possible death sentence if convicted. His co-defendant, Steven Hayes, was sentenced to death last year.
Komisarjevsky did not intend to kill the three, said his attorney, Walter Bansley.
"The evidence you are about to hear will shake your confidence in humanity," Bansley said. "The deaths that occurred were senseless, unnecessary and tragic."
Komisarjevsky wore a short haircut and a dark blue suit for the first day of the trial, which is expected to last up to three months including a potential penalty phase. The sole survivor, Dr. William Petit, sat in the front row with family members.
Authorities say Komisarjevsky and Hayes, two paroled burglars, broke into a Cheshire home in July 2007, beat William Petit and tied up his wife, Jennifer Hawke-Petit, and the couple's daughters, 11-year-old Michaela and 17-year-old Hayley. Hayes forced Hawke-Petit to withdraw money from a bank before he raped and strangled her in the family's home.
The girls, who had pillowcases placed over their heads, died of smoke inhalation after the house was doused with gasoline and set on fire.
Petit was also tied up but managed to escape to a neighbor's house to get help.
The bank teller, Mary Lyons, testified that Hawke-Petit was amazingly calm when Hayes took her to the bank.
"To me, she was trying to get done what she was sent into the bank to get done, so she could return to her family," she said.
She said that the intruders had apparently already gone through Hawke-Petit's wallet and when she opened it to look for identification, the only things inside were some pictures of her daughters.
Bansley admitted that it was Komisarjevsky who spotted Hawke-Petit and Michaela at a supermarket, followed them home and later returned to the house with Hayes to break in to get money. He admitted Komisarjevsky beat Petit with a baseball bat and tied up the family and that Komisarjevsky masturbated in Michaela's presence.
But he said the evidence shows Hayes raped and strangled Hawke-Petit, got the gasoline, poured it around the house and lit the fire.
Bansley said Hayes was worried about the DNA he left at the scene and told Komisarjevsky they would have to burn the house down and kill the family. He said Komisarjevsky was "stunned" and quoted from his confession to police that he told Hayes, "I'm not killing anyone. No one is dying by my hand today."
Bansley portrayed Komisarjevsky as panicked and indecisive, claiming he suffers from "cognitive difficulties" that makes him unable to make quick decisions in stressful situations.
During Hayes' trial, the defense blamed Komisarjevsky for escalating the violence and portrayed Komisarjevsky as the mastermind of the crime. They said Komisarjevsky had a long history of residential break-ins, attacked Petit with a bat, molested Michaela and pressured Hayes to sexually assault Hawke-Petit.
Prosecutors have repeatedly said both men were equally responsible for the crime.
On Monday, prosecutor Michael Dearington addressed the jury for only a few minutes, explaining the charges and the tragic nature of the crime.
Jeremiah Donovan, Komisarjevsky's attorney, noted Petit family supporters were wearing pins in memory of the victims. The defense has expressed concerns about such displays, saying they endanger their client's right to a fair trial.
Donovan called the supporters "the Petit posse," sparking a prosecutor's objection.