Why is Winston's betrayal of Julia such a key point in 1984?
When Winston is caught by the government, he is told that there are three stages to his "reintegration": learning, understanding, and acceptance. Winston doesn't betray Julia until the last stage and when he yells for the torturers to, "Do it to Julia! Not me!", he has capitulated and accepted Big Brother. The acceptance of Big Brother is essential to the government because it means that Winston no longer has free will; he is nothing more than a pawn of the government. Winston had thought he was strong and that he loved Julia. The government does not want any of the people to love anyone more than they love Big Brother and the government because that means that the individual might favor the one loved over the government. By betraying Julia, Winston shows that he loves the government, and Big Brother, more than anyone else.
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There are two parts to this.
The first would be when he finally decides for the torture to inflicted upon Julia when the rats were released onto him, to force him to hold on to himself first, before forcing that state of individuality to be accepting of the Party's manifesto, making him loyal and devoted to the party only.
The second would be the fact that he would resultingly place his loyalty, love and devotion to the Party, and only for the Party.