Best explained this way: a stray gamma ray could inspire you to murder someone, but you still get life in prison.
I don't know what you mean by this.
The argument from materialism against "free will" is often taken to mean: since free will doesn't exist, and all our actions are just a result of environmental factors and the chemical reactions inside our body, we can't really be responsible for anything we do as those neurons were going to fire that way no matter what based on their configuration before the murder, and before that, and...
example of this interpretation
I find this interpretation funny, because there is no relation between its premisces and its conclusion.
A faulty computer chip is faulty, and you must remove it because it's dangerous/bad. Likewise a bad individual is bad because he do things we deem bad therefore we give him invectives not to be bad, try to correct him or remove him from society.
The only case where absence of free will absove you from responability is when you're judge by an omniscient, omnipotent creator, since he created you that way, knew it, and could have corrected you. For human justice it doesn't matter at all.
Free will to an omniscient, omnipotent being is easily disprovable by the way : logically, any of your decisions is influenced by three things
A) Initial conditions (genetics, soul if you believe in it, ...)
B) living experiences (whatever happen to you)
C) Hazard
You are responsible for none, hence you cannot have free will.