FORT WORTH -- Is life in some urban neighborhoods so grimly dangerous that frightened people are justified in gunning down their neighbors?

Lawyers representing a teenager named Daimion Osby plan to rely on that explosive argument in a murder trial that began here yesterday.

Osby admits he shot two unarmed men, neatly and quickly, with two bullets. His attorneys say he did it because of "the fear that black people have of other black people" in the nation's most dangerous urban areas.

If this unique legal theory -- dubbed "urban survival syndrome" -- succeeds in Osby's trial, experts on crime worry it could fundamentally change the rules of justice in inner-city communities.

"It is an unsettling and perhaps precedent-setting case," said Bob Kahle, a criminal justice expert who teaches at Wayne State University and heads a Detroit crime-research firm.

"There are neighborhoods where people do prepare themselves for battle, to be quite honest. Not just in Fort Worth, but in major urban centers throughout the country," Kahle said in a recent interview. "But in a legal trial, people have to be held accountable for their own behavior."

Prosecutors in the Osby case contend that the defense seeks essentially to legalize black-on-black violence. Residents of the predominantly black neighborhood where the shooting occurred were outraged by Osby's defense when he was first tried for murder last spring.

"To suggest that the African American community is so enveloped by violence that everyone with sense leaves home with a gun is blatantly racist and insulting," said the Rev. Michael Bell, who led Fort Worth's black ministers at a news conference protesting the defense strategy during the first trial.

But the argument worked, say defense attorneys Bill Lane and David Bays. The first trial ended with a hung jury after a juror held out for acquittal.

Tarrant County District Judge Bill Burdock is allowing the argument to be made again, though he acknowledges it troubles him: "I just really don't know what our society would be like if this is what the law's going to be."

What happened on April 18, 1993, in this north Texas city of about 450,000 unfortunately happens all the time, all over the country.

"Our guy just didn't show these fellows the proper respect ... and they just increased the level of violence until he was fearful for his life," said Bays, who coined the term "urban survival" after consulting a race-relations expert he heard on talk radio.

In such circumstances, the defense argued, carrying a gun with a hair-trigger is the rational thing to do. Or, as Lane told the first jury, a weapon is like an American Express card in Osby's neighborhood: You don't leave home without it.

Even worse -- according to Jared Taylor, a race-relations expert and author from Louisville -- Osby's assailants were young black men, whom Taylor called "the most dangerous people in America." Taylor, a white man, plans to testify at the trial that he interprets FBI statistics to show that black males are far more likely to commit violent crimes than any other group separated by age, race and gender.

"This must be taken into consideration in judging whether or not it was realistic for Daimion Osby to think this was a kill-or-be-killed situation," said Taylor, author of the 1992 book, "Paved with Good Intentions: The Failure of Race Relations in Contemporary America."

Osby was 17 at the time, a 10th-grade dropout with no adult court record. The victims, Willie "Peanut" Brooks, 28, and his cousin, Marcus Brooks, 18, had been harassing him for more than a year, since Osby won $400 at an illegal Fort Worth gambling shack.

Willie Brooks "wanted me to give him action back to win his money," Osby said in an interview at the Tarrant County courthouse here, where he's been held since the shootings. "I told him it wasn't his money. I just walked out."

But this was his statement to police:

"There wasn't no words said. Marcus just come up to me and hit me in the face with his fist. We just started fighting. I just backed up and ... I pulled the gun out of my pocket and shot Marcus one time. Peanut was right there next to Marcus. I shot Peanut one time.

"I was just scared and started shooting."

Osby surrendered within hours. Police found a small, pearl-handled revolver in the pickup truck, but no weapons on the dead men.

They filed a charge of capital murder. Prosecutors did not seek the death penalty. If convicted, Osby, now 19, faces a life term with no chance of parole for 35 years.