Zak Bolland was high on cocaine and alcohol when he attacked the house, and claims he believed it was empty at the time.
The 23-year-old says his co-accused, David Worrall, had first suggested the fire bombing attack in Walkden, Manchester Crown Court heard.
Bolland and Worrall, 25, went to Michelle Pearson’s house. Her front door was smashed at about 12.30am on 11 December – the latest attack during a feud between Bolland and her son, Kyle.
The defendant blamed them for an earlier attempt to set fire to his mother’s home a couple of minutes’ walk away from the Pearsons’ house.
About four hours later, he and Worrall returned armed with two petrol bombs which they threw into the house.
One landed near the stairs, blocking the only exit to the ground floor and trapping the victims upstairs.
As flames engulfed the three-bedroom, mid-terrace home, Demi Pearson, 15, her eight-year-old brother Brandon and seven-year-old sister Lacie perished.
Mrs Pearson, 35, was rescued along with her youngest daughter, Lia, three, who died in hospital two days later.
Kyle managed to escape.
Bolland, his girlfriend Courtney Brierley, 20, and father-of-one Worrall all deny four counts of murder.
Bolland gave evidence on Friday as about 10 members of Michelle Pearson’s family sat in the public gallery watching him intently.
He said in the early hours of 11 December, after he had been drinking and taking cocaine, he got “wound up” so went round “kicking off” at Mrs Pearson’s house to find Kyle.
He believed police officers who had been called to the house were “taking them into protection”, he told the jury.
Bolland accused Worrall of suggesting retaliation for the apparent attempted arson on his mother’s home.
He told the jury: “I just thought it was the right idea and said ‘Should we do it?’ and Dave said ‘Yeah’.”
They bought £1.50 worth of petrol from a local garage and went to Mrs Pearson’s home.
Bolland said Worrall smashed the kitchen window with an axe, he lit toilet paper stuffed in the neck of a Budweiser bottle filled with petrol and Worrall threw it in and ran off.
He then tossed a second lit vodka bottle inside which exploded.
“I heard like a big whoosh. I didn’t look back,” he said.
“I did not think it would have been occupied. I assumed police had taken them into protection. I was drunk, high on cocaine, just rushing.
“I was just doing back to them what they did to me.”
Peter Wright QC, defending Bolland, asked him: “Did you intend to kill anyone?”
He replied: “That was not the intention whatsoever. I just intended to cause damage to the property.”
Hours later his mother called him to say the children had died and the police were looking for him.
Bolland said: “Devastated. I did not believe it. I had my head in my hands. I put my phone down and told Courtney. She was just in disbelief like I was.
“I did set the fire but I didn’t mean to harm anybody.”
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Bolland, Worrall and Brierley also deny three counts of attempted murder.
Bolland has admitted reckless arson, a charge denied by the other two.