Jason Betts, 38, was found unconscious with a head injury at his flat in Celestine Road, Yate, on 5 March last year and died later in hospital.
Nathan Skeen, of Ermine Road, Lewisham, was found guilty of manslaughter and actual bodily harm following a five-week trial at Bristol Crown Court.
The 29-year-old will be sentenced at a date to be fixed.
At an earlier hearing, Nixon Apire, 31, of Great North Road, Barnet, pleaded guilty to wounding with intent.
Avon and Somerset Police said the two men had been responsible for assaulting Mr Betts for "well over a week before his death".
In that time, he was stabbed in the leg, had cigarettes stubbed out on his face and was hit over the head with a bottle and a glass.
But Det Supt Andy Bevan said it was in a subsequent assault carried out by Skeen that Mr Betts suffered the fatal injury.
"Jason was a vulnerable man who had significant health and drug addiction issues," he said.
"Skeen and Apire callously took advantage of Jason's vulnerability and coerced him into selling class A drugs on their behalf in the Yate area.
"They had a suffocating control over Jason and when they felt he owed them money, they exacted their revenge by assaulting him."
A London drug dealer has been convicted of manslaughter after attacking an addict in Bristol for owing him money.