Where is the middle ground?

I watched the psychological thriller "Se7en" several weeks ago. It is about a serial killer whose murdures correspond with the seven deadly sins.
It is the most disturbing thing I have ever seen. And I do not think it is just or right in any way to kill someone sadistically and mercilessly because they emulate a deadly sin.
One of the last lines really stuck with me though. One from the killer. He said we tolerate these things. He talked about how we accept the sins. A man so fat (gluttony), that if you saw him eating in a restaurant, you would lose your appetite. Or a man that has committed his life to making money by lying (lawyer-greed). Or a prostitute, which he called a "disease-spreading whore".

So my point is, he is right, that we should not tolerate these things simply because we know the world is an imperfect place, but killing is not the answer.

How can these extremes, in terms of killing and emulating the sins, be avoided? How can people be changed for the better?

It would be so easy to say, "You can't". It would make perfect sense to give in and say that it is too difficult-if at all possible-to change people. But that is what that sadistic man was talking about.

I wish there was a way to start over. To prevent prostitution from ever existing. To prevent revenge from occurring. To prevent laziness from being possible.

But people cannot, and should not be controlled. No matter what, it is never okay to take away someone's free will-a God given gift itself.

So this begs the question: Is there a middle ground?
If so, where is it?