Nothing we do is done for no reason. Something is always the cause of our actions. Our will, free as it may seem, is actually determined by antecedents. If it wasn't its operation would be utterly random. One can't really do anything more than what the accumulative prior events lead one to do. So, if all our actions have an unavoidable cause can we really be held responsible for what we do?
Ethics and morals are simply convenient means of evaluating actions that cannot be otherwise.
Ah, a reductionist! I used to wonder about this myself.
Going past just our lives, you can look at all of the reactions in the universe as predictable events. When you pour two solutions together in a lab, you can estimate based on your limited knowledge how many atoms are in each beaker and what the reaction will be, but the fact is that there is a definable number of atoms with definable charges and, if you could perfectly replicate every part of the experiment, you could theoretically get the exact same result every time you did it. The idea is that if one could know where every single positive and negative charge in the universe is and how fast they are going, one could predict everything that ever has or will happen.
Apply this to biological functions, and poof, no free will. Everything is predetermined, every action you take calculated and described before your ancestors even crawled gasping out of the ocean.
Logically, free will is a myth.
Except....
There's this thing called quantum physics, which says that even if you could replicate your beaker mixtures perfectly, you would NOT get the same result every time. It says that there is a level of randomness inherent in the very molecules that make the world. Nothing can be certain, only predicted based on probability. Everything has a probability it will occur, instead of a calculated certainty, including our behaviors. So, if one could calculate the a person has a fifty/fifty chance of going or staying, then whether they do one or the other... that's choice. That's free will. And yeah, people are going to hold you accountable for it.
(I've only read a couple things about quantum physics and chaos theory and such, so this is just my interpretation of it, but I think I understand the basic premises.)