Why does winston from the book '1984' need to be vaporised after everything in room 101 etc?

By the time he had left the ministry of love after months of endless tortur and then being faced with his biggest fear (rats), he had been completely brainwashed by the party into losing all his 'bad' thoughts and actually loving big brother. Why then would he need to be vaporised if he no longer poses a threat to big brother?