Karuppusamy died on the spot in the attack last night, they said.
Police said the sub-inspector had come to the village along with Karuppusamy to question his daughter-in-law's father whether he had threatened to murder him (the DMK functionary).
On seeing Karuppusamy, the father and his sons attacked and hacked him to death in the presence of the SI, police said.
Search is on to nab the accused who fled after committing the crime, they said. PTI SSN BN