How does one need to understand desires in order to have them? I don't need to understand the biology and neuroscience behind thirst to know I am thirsty and aim unimpeded to fufill that desire. How is that not free?
I agree that our reasoning capabilities enable us to understand our actions, their consequences, and morality. I also agree we need to socially hold others responsible for actions and hold ourselves responsible. However, this is for nothing more than a utilitarian function morally and a reinforcement/punishing force psychologically. In reality, no one is morally responsible beyond the responsibility we place upon them for moral reasons.
The murderer is not truly responsible for his action. He has merely been dealt a bad hand in life: a mix of genetics, past and present environment, and mental states created by those genetics and past and present enviornments, that ultimately led him to murder. Our punishment should only aim to reeducate and rewire the murderer to a more proper conduct and to protect society from a force for harm. It cannot be to punish him for revenge or retribution.