The instructions are a product of both the prosecution and defense and largely just lay out what components the state is supposed to prove in order to satisfy the requirements of each of the charges. Those requirements are not as simple as just what's in the law and often are modified by court cases which is why they exist.
I do think juries have a role in using nullification judiciously but it's also important to remember the most prominent and wide spread use of jury nullification in history was during the Jim Crow era when juries routinely refused to convict or charge (in the case of grand juries) lynching cases.
That seems like a reasonable explanation of legal instructions, definitely more clear than my earlier one.
IMO the use of jury nullification really shouldn't depend at all on how it was used in the past though. Plenty of things have been used for terrible causes in the past, we can't throw the baby out with the bath water.
I do think juries have a role in using nullification judiciously but it's also important to remember the most prominent and wide spread use of jury nullification in history was during the Jim Crow era when juries routinely refused to convict or charge (in the case of grand juries) lynching cases.