Surrendering his own free will means that Winston will no longer be a threat to the governing forces. After giving up his free will, he's become a broken man and no longer be able to make his own decisions. He becomes emotionally and physically dead.
When Winston asks why he's being tortured, O’Brien says, “But we make the brain perfect before we blow it out.”
O’Brien means that they cannot have Winston die believing that the Party is false. They must torture him till he surrenders to the Party with his own free will, and he must not believe any longer that 2 + 2 = 4. He must truly believe that 2 +2 = whatever the Party says it is.
O’Brien tells Winston that he is a flaw in the pattern, a stain that must be wiped out. O’Brien says this because they cannot have someone die without being “washed clean".
Every person who believes what the Party teaches and says is rubbish must be convinced they were insane to believe that, and must die with their minds “clean” and loving Big Brother and the Party.
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