I'm unsure why some people deny the gift of free will. I guess because indirectly it allows them to deny the existence of a Creator who gave us that gift because they hate all things religious. It may also give them the ability to do whatever they damn well please by using the excuse they couldn't help themselves (i.e. weak minded). Criminals try taking advantage of this quite frequently. Many people like to view free will such that they have choices concerning whether or not to do something good but that free will magically stops at those actions which are evil. Again, that merely just proves they would prefer to satisfy their own agendas and bias to avoid punishment, retribution, etc. for their bad/evil/immoral/unethical actions brought on by their choices. But in the end, if you don't believe in free will then what's the point in having any moral code whatsoever? If you don't believe you can control what your mind tells your body to do then you should be able to argue that you have no reason to be held accountable for any wrongdoing, whether deemed wrong by you or society. Therefore what's the point in deeming something moral if you can't be held accountable for the immoral?
For those who deny free will, I ask you to prove it. Every decision you make on a daily basis proves you have free will. The mundane decisions in our lives don't disprove my statement. The fact that someone who knows you may be able to guess what your decision will be for any given choice doesn't force you into still making a specific choice, because you may still change your mind at the last minute. For the times when your friend is right when guessing what your choice will be it simply shows they know your tendencies and can infer based on their knowledge of you what you will choose. But *you* still have the final say. This is true despite your current emotion and is true despite your DNA, contrary to what criminals and homosexuals (notice 2 *separate* categories: criminals and homosexuals) would have you believe.
Free will is our greatest gift and it is also one of the fundamental properties of being a human being that so many people would prefer to ignore or outright deny as fact. Let me put it in the most basic terms possible for the laymen who choose to deny it: if we, as a species, didn't have free will, none of us wouldn't have the ability to choose when to wake up in the morning, when to eat breakfast, when to go to work, when to take vacation, when to take a new job, when to buy a new car/house, who to work for, with whom to go on your first date, who first to kiss, with whom to first have sex, whom to marry, etc. You may think that you can disprove the existence of free will by merely believing in [the Christian] God long enough to argue that if He is all powerful and has a plan then He is what defines what happens to us but you would be wrong again in your incorrect understanding of Christianity because even if He, for example, presents us with a new job opportunity He still leaves the final decision to us. He can't force us to accept the new job. Just as well, if Satan tempts us with drugs or money, we have the same free will to accept or deny those material desires. The mere fact that we have decision-making capability dictates we have free will.
More importantly, free will is completely separate from instincts and that is why humans are not animals. Of course, many people would prefer to deny that because again, it would lead to a mental conflict that involves having to accept the existence of a God that created Man and beast as separate entities rather than a natural process called evolution that created both as one in the same. Animals can only act on instincts; Man however, can choose whether to kill based on his moral compass. Only Man is held accountable for those same actions. A lot of people *hate* that they are held accountable to a higher power so they simply deny those concepts/theories/etc. that lend credence to a Creator and instead believe in those concepts that support their personal worldview of how to live without a higher authority that will eventually judge them, having naively rationalized that their choice (GASP! free will strikes again) to not believe in a God somehow would change God's rules. Satan gains a little more power every time a human believes they can outwit the system because that's one less person to follow God. Does failing to outwith the "system" (i.e. God's plan for us) mean there is no free will? No, it simply means one has chosen (by free will) whether to take God's side (and abide by His plan and rules) or to take their own path as influenced by Satan by outright denying God, creating their own moral code, denying they have no free will to do what's right/wrong, etc. Anyone who is against God is implicitly for Satan, whether or not they believe in either one.