In 1984, in the Ministry of Love, one was killed under what circumstances?
The conditions under which one is finally killed is after torture is absolute and complete. A full acceptance of the Party's doctrine must be attained.
In Book 3, chapter 3, O'Brien assures Winston that he will indeed die by shooting. In the previous chapter, O'Brien would not admit to the answer because he claimed Winston already knew the answer about the absolute punishment for the thought crimes Winston had committed.
Winston had to get through the phases of learning, understanding and acceptance in the Ministry of Love in order to be returned to the world and ready for his shooting. Throughout these phases, Winston is stripped of logical reasoning, human feeling, and all decency. Finally, in the end, Winston earned the bullet for his thought crimes:
The longhoped-for bullet was entering his brain.
Winston had longed to die throughout the torture, but the Party made him endure it until they destroyed his brain first.