The British National DNA Database is being built up by their government (which includes material from many innocent people), already allows the identification of those with milder predispositions to anger and violence. How soon before those people are subject to special surveillance? And if the state chose to carry out such surveillance, recognizing that the people in question may pose particular risks merely because of their biology, it could hardly then argue that they were wholly responsible for any crime that they did go on to commit.
Nor is it only the criminal law where free will matters. Markets also depend on the idea that personal choice is free choice. Mostly, that is not a problem. Even if choice is guided by unconscious instinct, that instinct will usually have been honed by natural selection to do the right thing. But not always. Fatty, sugary foods subvert evolved instincts, as do addictive drugs such as nicotine, alcohol and cocaine. Pornography does as well. Liberals say that individuals should be free to consume these, or not. Erode free will, and you erode that argument.
In fact, you begin to erode all freedom. Without a belief in free will, an ideology of freedom is bizarre. Though it will not happen quickly, shrinking the space in which free will can operate could have some uncomfortable repercussions.