The jury isn't there to interpret the law - plenty of people in the room doing that already. Juries are there to inject mercy. That's why they must be a 'jury of peers' - so they understand why and can intervene with mercy if necessary.
Juries are instructed to determine the facts of the case beyond a reasonable doubt, and thereby determine the verdict. Mercy is not part of jury instructions. A jury would be a pretty blunt instrument in the delivery of mercy, because they don't control sentencing, but only the verdict. All the jury can say is "guilty" or "not guilty" -- not something like "guilty, but give the guy a merciful sentence because of mitigating factors."
Thomas Jefferson said "I consider Trial by Jury as the only anchor yet imagined by man, by which a government can be held to the principles of its constitution".
Juries are allowed to vote "Guilty" or "Not Guilty" regardless of whether they think the accused broke the law. They cannot be punished by the Court. This is all expressly designed to allow Juries to correct inevitable tragedies brought about by too-literal (or too-political) interpretation of the law. I.e., mercy.
I suspect you are referring to jury nullification, which, contrary to proponents' beliefs, was never broadly accepted, nor designed intentionally as a feature of juries. It has always been contentious. It was certainly not the original purpose of juries. Furthermore, juries were punished very harshly for it in the past.
