The theory goes that, on an atomic level, interactions between particles is a deterministic process - it's wholly and completely predictable because the outcome is determined by a knowable set of rules. Given the fact that our brains are just[1] a huge set of particles and chemical processes it should then follow that a perfect computer model of someone's brain would make the exact same decisions as that person.
In other words, we don't actually make any decisions for ourselves. It's all determined by chemistry.
If that's true then someone could argue that the path of their life was predetermined by the state of the universe at any given time, so the fact they shot their wife was actually nothing they could have had any control over, and it's very unfair to lock them away for 30 years if they couldn't have changed anything. It's just the latest outcome of a chemical reaction that started at the Big Bang.
It's actually a very compelling argument. The counter argument requires 'magic' in the form of something that we can control that can't be measured. That's a big ask for any rational scientist.
[1] Pretty huge 'just' there because it discounts God, souls, etc. Many people think that the brain isn't just atomic particles. But there's no evidence that they're right. We've cut brains up and looked.