“I don’t want them to nullify the murder laws,” he said.
“I’m a big law-and-order guy when it comes to real crime.”
But, he said, there were other laws he wanted to nullify,
like drug and gambling laws.
You can't get one without the other. There is no murderer that some don't consider unjustly prosecuted. For instance there are a lot of people that would use nullification if told about it when trying someone accused of killing an abortion doctor.Worse, once you tell a jury they can judge the law you will get juries that go for anti-nullification--that is, they will convict the defendant even though they do not believe he broke the law. In this case at least the judge or an appeals court can reverse the conviction...if they can tell it happened. In many cases they won't be able to tell. In most cases, the prosecution alleges a set of facts that if true support conviction, and the defense alleges that those facts are not true. There is no way in general for the judge or the appeals court to distinguish between "jury believed prosecutor's alleged facts and so convicted" and "jury believed defendant's view of facts but convicted anyway".
Another problem is different outcomes for similar defendants. If 5-6% of people think that a particular law should not have passed, and a unanimous jury of 12 is required for conviction, then 50% of guilty defendants get a jury that will not convict.
It's really hard to design a system of checks and balances where you tell juries about nullification but somehow stop the bad outcomes I described above. The best anyone has been able to come up with is to not tell the jury about it. The idea then is that it is only applied in cases where some juror decides that his conscious would find convicting so wrong that he must violate his oath.