Lions kill their young. Or, rather, when an arriving male lion wrests control of a pride of lionesses from another male, he often kills the cubs that were fathered by the outsted male. The new father kills his own stepchildren.
In human society, we have many instincts as well, retained from our animal ancestors through evolution. If someone had the instinct to kill his stepchildren (and it seems likely that at least some humans retain these genes, due to genetic diversity), would he be responsible for his actions?
The answer is, of course, yes. Genes are not something we have, they are something we are. They define us. If someone had the genes to motivate him to kill his own stepchildren, it would feel to him that he was expressing his own free will when he carried it out. It would feel natural, just like a lion, because that's the way his brain would be wired to behave.
But more likely, an instinct to kill would be in conflict with other emotions and feelings he had (like guilt). Humans have more instincts than animals, not less, and these often conflict with each other.
However, the less conflicted our instincts, the more they feel like free will.
Our moral laws are written in this context of free will. Everyone is wired to behave differently, with a different set of instincts, numbering in the hundreds. (What are motivations, if not another type of instinct?) Our laws were written by people in the context of the free will around them. Laws label some expression of free will as good, and other free will as bad. If we kill, we are punished, because this is how one instinctive set of creatures (the law writers) wants to interact with another set of instinctive creatures.
I like to use the example of quantum physics as a metaphor that "you can't step outside the system". Laws were written by genetic (instinctive) beings, to judge the behavior of other genetic (instinctive) beings. There is no privileged position outside this system from which to judge, unless, of course, you are God.