Pillai, who was attached to Parippally police station, was stabbed to death by Antony during a vehicle check in June 2012. Another police officer was seriously injured in the attack.
While pronouncing Varghese guilty, District Principal and Sessions Court judge George observed that charges of murder and attempt to murder against him have been proved.
The court will give the verdict on the sentence on July 22.
After the murder, Varghese alias Aadu Antony, who was involved in more than 100 cases, went into hiding and he was arrested by officials of a special police team constituted to track him in October 2015.
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He was picked up from Gopalapuram in Tamil Nadu-Kerala border from the house of one of his wives.
Prosecution case was Antony stabbed to deathPillai, who tried to stop him when he tried to escape after grievously wounding a police officer on June 26, 2012.
The incident occurred at Kollam when police had stopped a suspicious vehicle and were conducting a search.
Varghese, a native of Kollam,had committed thefts at houses, offices, schools, banks and other establishments.
He used to steal mainly electronic goods, especially laptops and TVs.