Say I write a computer program, I write it to complete an equation. I then give it several different types of equations to solve and expect it to always have the same result. My analogy fails when we come to punishment, because nothing is less just than infinite punishment for finite crime. The worst person who ever lived or will live could not possibly deserve that.
So we've got the celestial "screw you" down. Let's talk about God's plan for us. God is supposed to have this divine plan for each of our lives. That is not possible if we have free will. Imagine you are commanding an army. The catch is, none of your soldiers follow orders unless it suits their personal agenda. Can you imagine commanding a free-willed army? The only thing which holds a plan involving multiple people together is knowledge of how they will act. If they change what they are to do, they can endanger the entire plan and screw it up completely. The typical counter-argument to this is that God can work all things that happen into his plan. The problem is that it's still a plan, which requires manipulation of at least several elements.
Next, divine intervention. Some people think God intervenes on a daily basis for them. If God saves parking spaces and the like for the individual like some believe, he must deprive another of that parking space. The other person would have to be swayed divinely to park somewhere else or not make that particular stop. What does that do to free will? I suppose God would have to make them want to not stop or not park there so that they'd freely choose to avoid it, but can that be considered free choice? The educated person cannot believe in both the divine and free will simultaneously.
A friend of mine recently posted on facebook "If I had a time machine, I would go back to the Precambrian, throw a pebble in the ocean, and then fast forward to the present to see how much had changed.". The result in this experiment could be catastrophic, or it could have almost no effect. In the same way every time God meddles, he disrupts the entire natural order. When he does that he could change almost everything and what does that say about free will?
As a Christian, there can be no free will. Without free will we're not responsible for our actions. That removes sin, making us sinless. We'd have no need for God to "save" us. No hell to punish us for the sin. If one argues that we're still responsible without free will then God is a tyrant (which is a notion I wouldn't be above, I derive great pleasure from knowing that he doesn't exist).