David Friedman hosts an interesting discussion on jury nullification on his website.
I was recently involved in an exchange with a prominent jurist on the issue of jury nullification—the doctrine that jurors are entitled to nullify bad laws by refusing to convict a defendant who did something that is illegal but, in their view, should not be. He pointed out, correctly, that it is a very dangerous doctrine. If everyone believes in jury nullification and one person in five believes that it is all right to murder abortionists, someone who murders an abortionist is unlikely to be convicted. Similarly for any other target group that a significant minority believes deserves death.