It's "part of the law" we say a hole because it sits between 2 actually written laws. Jury nullification is not arcticle 5.x.2.y of Z legislation.

The law says if you shoot someone else while not in imminent danger for life (self-defense), you break that written law (and no other supercedes it). The jurors can find that while he killed someone and wasn't in immediate danger, this law doesn't apply to him because . That's jury nullification, they nullified the law for reasons not already exempt in the law while agreeing he did the thing we normally call murder beyond a reasonable doubt.

You can swear to uphold the law and nullify it, thats why jury nullification works and why they aren't found to have purgured themselves. But if you try to nullify the law, knowing he is guilty by the rules of the law for other motives, that's not upholding the law and could be a crime. Luigi would have to be found not guilty because "murder" is not what he did (which is totally plausible and needs to be argued)