Well that's the conundrum I always face when thinking about the crucifixion. I mean, we have to accept the means God used to redeem us (human incarnation, suffering, and death). I wonder if the way He chose was 'necessary' based on the nature of how man fell and what man was/is, or if he could have just come down as a big old ball of light and said "I pushed the reboot button - you're all okay again"?
I admit it's hard understanding any evil as necessary yet not a denial of free will. I have to read up on it myself. I know in Exodus it says that God made pharoah obdurate show Moses could display the glory of God.