The incident occured at Damacherla village under Wadapally Police station's limits, this morning.
The victims were identified as B Venkateshwar Reddy (58) and Parvathamma (37). Reddy was a farmer.
Guruva Reddy, one of his three sons, was apparently angered by his father's illicit relationship, the investigating officer told PTI over phone.
This morning, Guruva first tied his father and the woman to a tree and smashed their heads with a pestle before pouring kerosene and setting fire to them, police said.
He is being questioned, but has not been arrested formally yet.
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