Complex sentence: Initially, they had a argument with him reminding him had been repeatedly told not to write anything about Verma, then they pinned him down, poured petrol on him and set him on fire.
Split into simpler sentences: Initially, they got into an argument with him reminding him that he had been repeatedly told not to write anything against Verma. Then they pinned him down, poured petrol on him and set him on fire.