When is it right to do wrong?

The murder of George Tiller is, to me, an obvious and reprehensible crime. But what's intriguing is that not everyone sees it that way (and not just the person who pulled the trigger). Which got me thinking about something I've always wondered: what separates legitimate resistance to a societal evil, from an evil crime committed against a legitimate societal practice?

Put differently, why is George Tiller's murderer a criminal, and John Brown a martyr? Tiller's killer murdered one person out of religious conviction, and was presumably convinced he was saving lives. Brown and his men killed a dozen people out of religious conviction, in an attempt to end slavery.

To a lot of people, merely asking that question is offensive - slavery is an unquestionable evil, but abortion is a woman's legitimate right should she choose it. I believe both of those things, but I wonder about what history will believe. I'm pretty sure the slavery question is settled, but abortion is still a fluid and controversial issue, and the people who oppose it consider it a form of genocide.

While I'm confident that abortion isn't evil, and should be permitted - safe, legal and rare - a lot of practices that didn't seem barbaric at the time come to look that way in hindsight. It's possible that we'll one day look upon abortion clinics (and slaughterhouses, in our vegetarian potential future) the way we currently do human sacrifice or cannibalism.