I personally rather liked what Scotticher said – msmith’s point that one is responsible for one’s actions before a court of law is significant here.
Murder is by definition a crime of motive – it involves premeditation, or at a minimum reckless disregard for human life in the commission of another felony. Otherwise it’s manslaughter.
Hence a dysfunctional childhood is a mitigating factor – not an exculpating factor. If the man is capable of knowing right from wrong in the legal sense, he can form the intention of committing murder, and carry it out. Therefore, he would be guilty of murder after trial. However, the presumption that the abstract “reasonable man” is able to grasp the right of his fellow man to live unmolested by him is weakened by the abused-as-a-child murderer’s presumable sociopathy – his weak grasp on the rights of his fellow man.
Take a parallel with a mentally deficient person – while he may be capable of killing another, he’s not capable of premeditation of the “in cold blood” sort. Therefore, his killing of another is punishable as manslaughter, not murder.
When one suggests mitigating factors, one is not attempting to “feel sorry for the criminal” in the sense of excusing his guilt; one is rather trying to weigh factors affecting what punishment is just (not saying that no punishment should be imposed). Our hypothetical sociopathic killer might have committed a rape-murder and dismembered the corpse, which would be an aggravating factor, also to be weighed into the judgment.
I recently read a bunch of devotionals written by a man in prison in Oklahoma for murder. He met almost precisely the description hypothecated in the OP, save that it was not a double homicide, and was committed in his late teens. Through quite a lot of work on the parts of himself and others, he came to grasp the evilness of his crime and his life to that point, and seriously reformed his life – expecting to spend the rest of it in prison, but intending to make the best of what was left as partial expiation for his crime. He volunteered to speak to groups of tough kids, taken there by guards, exhorting them not to make the mistake he did – and using their language to them. He was recently released on parole – did not expect it, in view of his crime, but the parole board was convinced (as am I and were the editors of Forward Day by Day, for which he wrote) that he was a changed man, and released him on a strict parole. When last he wrote, he was full of plans for the future and what good he could do in the world. I’m praying for him to be able to hold to his high intentions in “the real world.”
The typical murderer has removed one life from Earth’s resources of humanity. The question is, is it in society’s best interests to remove him as well, or is he one of those who can be turned to the doing of good rather than evil? It’s always a tough question, and easy to second-guess. But I use this case as a counter-example to the stereotype of the murder who is totally evil, with no good in him.