If your position is true, if such diamond clarity rings like the bell of truth around this concept, then there would be no damage whatever to the prosecuction's case if the defendent were given is right to explain his actions - and their reasons. After all, the premeditation part of the charge does, in fact, demand an explanation. And also after all, the law is so clear, so absolute under - how did you put it, "Natural Law or God's law" - that anything he would have given as an explanation would have had zero effect on his conviction.
But if that's the case, then why did the prosecutor and judge so strenuously object to him telling his reasons for the killing? Maybe they were worried that the stupid jury wasn't as sharp as you are about the absoluteness of the law? Or maybe - just maybe - the application of the law is not as utterly indifferent to nullification as you think it is? After all, even though you purport to "support in principle" jury nullification, it doesn't sound like you have much use for it. Even in this case, with an accusation of a known killer of thousands of full-term babies on one hand, and a murderer who was solely motivated by those killings on the other hand.
The application of jury nullification in such a scenario sure seems like an appropriate legal, social, moral, and cultural challenge to the validity of the law protecting the abortionist. In other words, an application of the actual REASON for the existence of the jury nullification process. In fact, jury nullification might have been so clearly applicable here that maybe - just maybe - the prosecution wasn't as absolutely confident in the proper application of murder one in this case as you are.
And, if he was allowed to speak, and the jury nullified the application of the murder charge under these conditions, then that would be lawful, wouldn't it? And the whole country would also learn about the power and purpose of jury nullification, wouldn't it?
Not that any of these consideration should impinge upon your ringing clarity of the meaning and application of "the law," of course.
It's perplexing, because if you take 2 steps back and look at the context, the whole situation is legally and morally deranged. Here we have this serial child killer, operating in a state, Kansas, where --- even given Roe vs Wade --- the late-pregnancy abortions he committed were in fact strictly illegal. He got away with it for decades, piling up thousands of corpses, because he had the support of the Kansas political establishment, up to and including the governor of the state. (Now HHS Secretary!)
And if you take another 2 steps back and look at the even larger context, you see that child-kiling has had the political protection of both political parties and the tacit acceptance of even those who say they are opposed it it, to the extent that over 2 generations now, 1/3 of our children have been mutilated and discarded. Nations of children have been murdered.
All this with periodic hand-wringing and speechifying and wailing (I include myself in the ranks of the feckless hand-wringers) ---but on and on it goes.
So this fellow Roeder, with good intentions and all the ringing moral clarity of a cracked terracotta pot, decides to just shoot the child-killer. Which, if approved via jury nullifiation, would open the door for ad-hoc vigilantes gunning down abortionists in every state --- why not? And why stop there? Many passionate people have no lack of good reasons to take their turn at moral self expression via homicide.
Deus vult, Allah akbar and Ecrasez l'infame. The end of the world in flames.
OK. So much for the context.
Should Roeder have been allowed to explain to the jury why he committed this act? Thus opening the door to jury nullification?
Or to the judge? (It seems to me it would be a mitigating factor ---I suppose this would be in the sentencing phase.)
Not being a lawyer and not being acquainted with Kansas law, I don't know when or to whom that explanation should properly be given. But it seems to me that there should be some time when such an explanations should be admitted.
If you twist my arm and make me choose, this is how I would choose: at the sentencing phase, to the judge.
May God have mercy on our souls.