A court in Kerala's Kottayam district on Tuesday found Narendra Kumar, a native of Uttar Pradesh, guilty of murdering three family members in Parampuzha in 2015.
The court will pronounce the quantum of punishment on Wednesday.
Earlier in May 2015, 72-year-old Lalason, his 54-year-old wife Presanna Kumari and their elder son Praveen Lal, 28, were found hacked and electrocuted, with acid poured on their faces. Their bodies were found at the laundry shop they were running in Parambuzha.
Narendra, who was working at the shop, was arrested by the police in Uttar Pradesh's Firozabad district.
The police nabbed Narendran, six days after the murder, by tracking his mobile phone which he had stolen.
A chargesheet in this case was filed in August last year.