Why doesn't God take free will away from people like Hitler?
If the ability to use free will is conditioned upon making good moral choices, then the will isn't really free. Our ability to freely choose to love and to serve God was of such importance to God that he allowed for the fact that not all would use their free will rightly. It is all the more important, then, for those of us who do choose to love and to serve God to do all that we can to oppose the evil actions of those, like Adolf Hitler, who use their free will to harm others.
I recommend a blog post I wrote about choice, free will, and the mystery of evil, Our Choices, Ourselves.