Why does the Party not kill Winston? In other words, what is a more successful technique on their part to use instead?

The Party
may actually killat the end of the novel, but we don't "see" it happen.  I would say
it does not kill him before that because it wants him to see the error of his ways and it wants
him to accept the supremacy of the Party willingly.

I do not think there is
any real reason why the Party...