1984 - George Orwell analysis question?

I was reading the book "1984" by george orwell and i got to book III ch. 3 without any problems but i have some questions: Why would the inner party put so much effort and time into torturing and converting the offender to the way of thinking of the party and big brother if they were going to kill them anyway and already had the power to make it so that he didn’t exist in the minds of every human in the world. If the party can control the past then there will be no others to rise up and take the place of the martyr. so why torture and spend all that time getting the point across that 2 + 2 might equal 3 or 5? And is the party really just seeking power and not concerned about the welfare of the people and citizens of Oceania? Is the party sane in their way of thinking? Is the idea that “if the party SAYS it never happened, then it never really happened” just being played out by the party? Will these questions be answered at the end of the book?