A judge's decision to reduce a killer's sentence because he has genetic mutations linked to violence raises a thorny question – can your genes ever absolve you of responsibility for a particular act?
Regards the idea of genes absolving someone of responsibility: If they do then I think the genes reduce a person's rights at the same time. If a person has genes that compel him to violate the rights of others then that person lacks attributes needed to make that person into a full rights-possessing being.
In my view human rights do not come about as a result of our having spirits or souls. We do not have rights because we all just up and decided we had rights either. A rights-based system requires that the rights-possessing conscious intelligent beings have the capacity to act as moral agents. Someone who is compelled to murder or steal lacks attributes needed in a rights-possessing being. The ability to reason is not by itself sufficient to make a being have the attributes needed to possess rights.
Cutting the sentence of a genetically driven killer by a year is nuts. If someone really can't prevent themselves from carrying out murder then that person needs to be permanently removed from civilized society.
In 2007, Abdelmalek Bayout admitted to stabbing and killing a man and received a sentenced of 9 years and 2 months. Last week, Nature reported that Pier Valerio Reinotti, an appeal court judge in Trieste, Italy, cut Bayout's sentence by a year after finding out he has gene variants linked to aggression. Leaving aside the question of whether this link is well enough understood to justify Reinotti's decision, should genes ever be considered a legitimate defence?
If a lion or tiger kills a human we do not consider it a murderer because we do not view lions and tigers and bears (oh my) as moral agents. I expect genetic research and neuroscience to continue to produce results that leave less room for free will as the agent for decision-making in humans. To the extent that compulsions and drives pull people toward engaging in behaviors those people become less rights-deserving.