To me, the rejection of free will seems so useless that it is not even worth seriously considering. However, the existence of free will does seem to necessitate a normative standard by which certain choices can be objectively justified. Can nihilism reconcile free will with the necessity of objective justification that free will seems to imply.
The question of free will certainly is important, especially when one considers the implications in situations of "moral luck". It may be argued that freedom does not allow moral decision making, but can the opposite be true? It would seem strange to suggest that freedom, somehow requires moral oughts to exist, considering how oughts are distinct from facts.
Compatibilists claim determinism can be reconciled with free will, or as Schopenhauer said: "Man can will as he wills but cannot will what he wills." Compatibilists contend that even if determinism were true, it would still be possible for us to have free will. Thus someone can be held morally responsible for their pre-determined actions.
Before discussing moral "oughts", it is certainly worth noting the is-ought distinction that Hume famously made in terms of objective morality, emotions must be separated from moral oughts. Nihilists believe these oughts do not exist, this does prevent them having a personal subjective preference to one behaviour or another.
Returning to your question; just because one can make free decisions, does that mean there is a way in which one ought to act? It appears that the two parts of these questions are mutually exclusive.
To me, the rejection of free will seems so useless that it is not even worth seriously considering. Free will does seem to necessitate a normative standard by which certain choices can be objectively justified. Can nihilism reconcile free will with the necessity of objective justification that free will seems to imply?
Rejection of free will isn't useless, determinism isn't nihilism, and we all need justification. Free will is a illusion.
“Men believe themselves free, simply because they are conscious of their actions, and unconscious of the causes whereby those actions are determined”. Spinoza
We do not know what we intend to do until the intention itself arises. To understand this is to realize that we are not the authors of our thoughts. Buddhism rejects the idea of an agent, and thus the idea that freedom is a free will belonging to an agent (Wikipedia). Buddhism and other Eastern philosophies have long ago cast off free agency. There are at least several million people on this planet who don't believe in free agency and seem to be living happy lives. Unlike plants, animals, we see our will as uncaused causes. It does not seem very plausible that the deterministic, causal relationships that existed in the universe for billions of years would suddenly suspend themselves to accommodate the relatively recent development of the brain’s nerve. How can a nondetermined choice provide people with any control over their behaviors?
What would free will give us that we don't already have in a determined world? Love and gratitude seem to be built into us as part of our evolutionary heritage as social animals; they are part of what holds groups together and are products of determinism. Moral sentiments may be viewed as a reward mechanism, to make someone more sensitive to distant rewards and punishments. Hate and resentment have evolutionary components as well. Belief in free will reinforces hate by ascribing arbitrariness to those that who elicit them, they are seen as deliberately choosing those actions. Skill acquired by luck is still skill, and a skillful violinist sounds much different than one who has not been lucky enough to develop to the same level. We can express appreciation and gratitude for our good fortune in hearing a virtuoso performance without granting the performer free will. People advise their fellows of possible ways to improve their behavior do so because we are social animals that aid their fellows have better survival rates.
Determinism don't say that the level of punishment we administer to defaulters is never determined by the extent they were in control of the events leading up their crimes. An efficient social contract won't call for penalties to be inflicted when the circumstances under which the crime was committed are such that inflicting the penalty won't deter similar deeds. If it can be demonstrated that Abel's death at the hand of Cain was accidental by pointing to the immediate cause of the crime, we don't need to follow the practice of our predecessors by holding Cain to blame. However there are plenty of exceptions, as with the continuing execution of mentally ill criminals in Texas. But when the causal chain is uncertain, our social contract needs to be ruthless. All of us, for example, are held to be guilty until proved innocent when it comes to paying tax. The extent of the damage suffered by a victim is often largely outside the criminal's control. For example, a mugger may hit two victims equally hard, but he will only be tried for murder if one of them happens to have an unusually thin skull. The doctrine that the seriousness of a crime is determined by the consequences to the victim also fits the same pattern: when the causal chain is uncertain, our social contract needs to be ruthless.
Again, a social contract that doesn't discourage antisocial behavior won't survive, and so deviants have to be punished. The driver who neglects to check his brakes and is guilty, if no harm ensues, of mere negligence. But if through bad luck the driver kills a child in his path, he is judged and judges himself more harshly, even though his input is the same. We recognize that being imprisoned is unpleasant, but we don't ask whether people got infected by accident or through negligence by scarlet fever before temporarily quarantined them. We isolate them, because the alternative is that the disease will spread, and we don't want healthy people to catch it. To say that I am responsible for a given act means that, because the act arose here in my body/mind rather then somewhere else, I am liable for whatever the consequences may be of this arising. Deliberate punishment makes sense only if it has some kind of deterrent or rehabilitative value. The more freedom we assume that agent to have had in its choosing, the angrier we are likely to feel over the choice actually made. With the giving up of agency, speculating about either the past or the future becomes less interesting. We are drawn more intimately into the present.
People will more readily attribute free will to themselves rather than others. They regard their own choices as less predictable and give desires and intentions the strongest weight for their own behavior, but rate personality traits as most predictive of other people. Any connection between conscious thought and action should be determined by scientific enquiry, and not by unreliable introspection and feelings. People consider acts more "free" when they involve making random actions for example. But notably random actions, may not be possible; when people attempt to perform tasks in a random manner their behavior betrays many patterns, such as generating random numbers. Hawking and Mlodinow suggest a thought experiment in which one encounters an alien that may be a robot. Is such a machine deterministic? We cannot predict the machine's exact behavior without a complete knowledge of its personal history with its environment, the reliability of its components, and its present state of programming, uncertainties in which limit us to probabilistic statements. An illusion of free will is created due to the generation of infinite or computationally complex behavior from the interaction of a finite set of rules and parameters. The unpredictability of the emerging behavior from deterministic processes leads to a perception of free will.
Neuroscience is valuable in developing models of how the brain may construct our experience of free will. In alien hand syndrome, the afflicted individual's limb will produce meaningful behaviors without the intention of the subject. Unconscious will is not will or “free”will. The sense of agency does not emerge in conjunction with the overt appearance of the purposeful act even though the sense of ownership in relationship to the body part is maintained. The clinical definition requires feeling that one limb is foreign or has a will of its own, together with observable involuntary motor activity. This syndrome is often a result of damage to the corpus callosum, either when it is severed to treat intractable epilepsy or due to a stroke. The standard neurological explanation is that the felt will reported by the speaking left hemisphere does not correspond with the actions performed by the non-speaking right hemisphere.
Some ideas and text from Wikipedia, Ken Binmore and Norman Bearrentine