You're missing the point.

The actual act of decision making is like a program running on a highly advanced biochemical computer. Your decision making process is a byproduct of interaction between your internal biology and your environment over the course of your life.

The decision you came to was the only one you were ever going to reach at that specific time frame. The choice exists, but the way in which you decide is determined by your brain, which functions based on nothing more than firing rates of brain cells.

Because of this, going on a killing spree was inevitable. It's not inevitable in the sense that nothing could have been done to stop it, it's inevitable in that the way everything aligned at that moment in time made it happen. The state of the person's brain function, the environmental circumstances that provided that person's brain with data and the choice to commit the crime, and the lack of intervention against it. Your brain is not some other-worldly magical object, it follows the same laws of physics and chemistry that every other object in the universe follows.

Likewise, society's reaction to people who go on killing sprees is also inevitable. We punish people who act in such destructive manners not because they made a poor choice (although this is how we justify it because we don't actually understand ourselves very well), we punish them because it is how we are conditioned to react to such behaviors.

This process is also deterministic but not fixed. The manner in which you decide things changes every second. So while you may be a perfectly normal person one day, if something rather odd is happening in your brain and a certain set of circumstances come along at that specific time, you may just go on a shooting spree. If these same circumstances happened at a time when your brain was functioning in a different way, you won't go on a shooting spree. This is why deterministic but not fixed is so complex, it means that no set of conditions will perfectly predict how you will behave at all times. Your brain is a machine, but one which constantly changes. Because of this, the decision making process of the human mind changes across time.

It's really messed up, and to be perfectly honest it's a lot easier not to think about it too much on a day to day basis. I personally know what's actually going on, but at the same time in order to actually function as a member of society I have to delude myself into thinking I'm actually making choices in a non-deterministic fashion. If I didn't practice this sort of 'double-think' I would probably go completely insane.

(Random aside, as if what I already wrote wasn't enough:The whole question becomes even more insane if you gain a basic understanding of quantum physics. Then you start thinking in terms of things being both inevitable and not inevitable simultaneously and all kinds of other very strange logical paradoxes. Luckily quantum effects are essentially negligible on any scale above the sub-atomic, and so can probably be more or less ignored, but it's interesting to consider.)

Anyways, that satisfies my intellectual masturbation for the day, I'll clean up now.