A jilted boyfriend accused of murdering his former girlfriend, her two siblings and their mother's partner had killed before, a jury has been told.
Mark Goldstraw denied four counts of murder
Marcus Carter, seven, Patricia, 10, and Samantha, 16, died with Roddy Hine, 44, in a house explosion in Cheddleton, Staffordshire, in March 2006.
Mark Goldstraw, 31, of Compton House, Leek, denied four counts of murder.
Stafford Crown Court heard he was convicted of the manslaughter of Deborah Wheatley in 2001.
He beat her around the head with a mallet when she decided not to leave her husband for him, the court heard.
'Possessive and jealous'
Christopher Hotten QC, prosecuting, said his previous conviction did not mean he was guilty of the four murders.
"In the presence of all the other evidence, is his previous conviction simply an irrelevant and unfortunate coincidence or does it help to explain why this man of all men would react with murderous violence when others would not have done?" he said.
The jury was told Mr Goldstraw had been in a relationship with Samantha Carter which had ended months before the fire.
Mr Hotten said the defendant had been a friend of Mr Hine and had looked after the family home while they went on holiday.
But he said Mr Goldstraw became abusive after his relationship with Miss Carter soured.
"He (Mr Goldstraw) was a friend and rejected boyfriend of Samantha, once upon a time a trusted friend of Roderick Hine, a frequent visitor to the house," he said.
The three Carter children died in the explosion
"But he was possessive and jealous and she decided to break off the relationship.
"This (the fire) had all the signs of a dreadful accident but we say beyond any doubt this was deliberate arson with murderous intent.
"We say he (Mr Goldstraw) made plans to take revenge upon Samantha and her family to deal with the rejection."
Mr Hotten said that in the weeks running up to the fire, Mr Goldstraw had tried to persuade Miss Carter to get back together with him.
He had even told her he had been diagnosed with a brain tumour in a bid to win her back.
The trial continues.