I was walking down Third Street on Saturday night with my boyfriend, Kevin, when a car hit a biker at the corner of Bainbridge Street. It was a minor accident— the car had stopped and the biker, who seemed fine, was talking with the driver—so we kept on walking toward the bar where he was DJing that night. Suddenly, we heard the gunning of the engine and an awful wail rang out as we spun around just in time to see the car knock the biker to the pavement and drive right over him, crushing his leg under the tire, the metal bike screeching against the concrete as the car dragged it out from under him. The driver just kept going, swerving down Third Street in a desperate attempt to flee the scene of the crime.

Before I knew what was happening, Kevin had bolted down the street and leaped onto the hood of the moving car, which startled the driver enough to make her hit the brakes. "Get the hell off my car!" she hissed. He stomped on the hood and kicked at the windshield. "Step out of the car! Now!" Moments later, I saw a plainclothes police officer waving what appeared to be a rather large gun at the driver, at which point Kevin hopped off the hood and ducked quietly out of the picture.

By the time the cops swarmed in, Kevin and I were nowhere to be found. He wanted to avoid police questioning—it made him nervous. Is it illegal to jump on top of someone's moving vehicle? Probably. Would the fact that he had been performing a good deed absolve him of that crime? We weren't sure. After all, as the saying goes, no good deed goes unpunished.

Curious, I began to look into the legal ramifications of vigilante good-deeding. "Good Samaritan" laws grant legal immunity only to those giving first aid to the injured. When it comes to stopping criminals in flight, no specific laws apply. Anyone is entitled to make a citizen's arrest, so long as the arrestee has committed a felony—and in Pennsylvania, leaving the scene of an accident is a felony if the victim is seriously injured. "Seriously injured," however, is an ambiguous phrase, and it's unclear whether stomping on someone's car meets the definition of a citizen's arrest. If accused of making a false arrest, vigilantes can end up in serious legal trouble, unprotected by the legal immunities granted to bona fide cops.

Our legal system simply leaves no room for the vigilante to explain himself, especially one lacking an expensive attorney. Judges want only to know if you committed a crime, not why. It's not surprising, then, that bystanders gawk at crime as passively as if they were watching it on television. Who wants to be a hero and get locked behind bars? Did the Metropolis Police ever cart Superman off in handcuffs?

We are living in an age where we can't always rely on the authorities (remember the "lawless looting" for food, clothing and medicine in New Orleans?), yet rule of law discourages citizens from relying on each other. Might the world ever learn to accommodate the kindness of strangers?

I don't think so. Sure, better civil immunity laws and equal representation for all would help. But sweeping laws will never cover every small situation. What's morally right and what's legal will not and cannot always coincide. The fact is, there's no way the law can fully protect acts of civil heroism, and the vigilante will always lurk in the shadows of the system.

Maybe that's a good thing. Vigilantism does have a dark side—lynch mobs, murderous pro-lifers, Bernard Goetz. If we left it up to the individual to determine what's allowed, we would be steeped in dangerous relativism. It's important to live by a set of established laws. But situations inevitably arise when you have to toss the law aside and do what's right.

That's why the world desperately needs people who are willing to act on moral urgency, on the outskirts of law, to live in anonymity and hope for the best. Superman shouldn't be arrested, but he wouldn't do much good as chief of police, either. Kevin did the right thing by jumping on that car, and the right thing by disappearing afterward. Still, the faceless vigilantes who roam our streets deserve a shout-out now and again.

Amanda Gefter is an editor at New Scientist magazine.