When Winston was captured, he was told by O'Brien that he was a flaw in the system. and he must be "cured", but he was also told that after he was made sane, he would be killed anyways.
Why was O'Brien so insistent on "curing" Winston? Why not just kill him as soon as he was captured? Wouldn't that have been a much easier solution? Or is this just a power-crazed party member, who is trying to prove that the party has control over everything, including people's minds?
