Why does the Party not simply kill Winston upon witnessing his crimes?

O'Brien, of the inner party, finds Winston challenging. O'Brien has been watching Winston for seven years. Winston's keen intellect intrigues him. He allows Winston to write in his diary and carry on his affair with Julia because this adds to Winston's experience. O'Brien wants the challenge of re-educating Winston. He wants to match wits with Winston and change him; he wants to "cure" him.