The easiest thing to say about Bernhard Hugo Goetz, who has identified himself as the Subway Vigilante, is that it simply won't do to have private citizens taking the law into their own hands. It may not be possible to work up any sympathy for the four teen-agers Goetz shot after they tried to get money from him, and, as is evidenced by the flood of support, it's perfectly easy to be sympathetic toward Goetz.

At least at first. But then you start asking yourself questions -- Was it really self-defense? Was it necessary to shoot the four teen-agers who, so far as Goetz apparently knew, were unarmed (they later turned out to have been carrying screwdrivers)? Wouldn't it have been enough to point the gun at them and send them scurrying, or to hold them at gunpoint while other passengers summoned the authorities? Was there an element of vengeance in the actions of Goetz, who had earlier been a mugging victim? Would New York City really be that much safer if most of its citizens went around wearing sidearms?

You think it through, and you are likely to find yourself drifting toward the conclusion that -- as glad as you might be to see any hoodlums get what they deserve -- Goetz's lawlessness falls short of genuine heroism.

But there is another aspect of vigilantism that is a good deal more problematic. What is the right thing to do when obeying the law does not produce justice but only reduces law-abiding citizens to patsies?

One of the clearest illustrations of the problem took place in July 1981 in Skidmore, Mo., when Kenneth Rex McElroy, a celebrated town bully, was gunned down on a crowded street. McElroy, by all accounts, was the classic bad guy. He had shot the town grocer and a local farmer, without apparent reason. He had been charged twice with rape of a minor, had threatened a local minister (who afterward took to carrying a gun for protection) and was widely thought to be a persistent hog rustler.

For reasons that require little imagination, it was always a problem finding witnesses against him. Charges against him seldom stuck. The town marshal resigned after McElroy held him at gunpoint for some 20 minutes, telling him that he believed in "killing any sonofabitch that tries to put me in prison for the rest of my life." It was McElroy's boast that no one could touch him.

But that July morning, someone did. McElroy left a bar where he had been drinking and was followed by somewhere between 20 and 50 men. He had climbed into his truck when someone (in spite of the number of apparent witnesses, the authorities never were able to establish just who) took a rifle and shot him twice in the head. Nobody was ever indicted for his murder.

If you can understand why no witnesses came forward to testify against McElroy's killer, you can understand the reason for the nationwide support for New York's subway vigilante. People support the law because they understand that society cannot work without it. But when people come to believe that the law no longer is capable of protecting them but only protects those who hold the law in contempt -- whether gun-toting bullies, street thugs or international terrorists -- support for the law disappears.

Whether in Skidmore or in New York City, people conclude that if they are going to live in a jungle, they had better adopt the jungle's rules.

Admonishing them not to take the law into their own hands makes sense only if the law is in hands that are capable of producing security and justice. Let the law become ineffectual, and the vigilante is elevated to the status of hero.

The twin lessons of the present case are the danger of vigilantism and the inability of the law to provide basic security. It's pointless to philosophize on the former unless we are also prepared to do something about the latter.