Six years ago, when Matthiessen started work on “a big Florida novel,” he thought that the story of Watson’s murder in 1910 would be no more than a small thread in the narrative. But no sooner had he started to write than the Watson story began to grow. “This thing just took over like a strangler fig,” he says.

Killing Mr. Watson (384 pages. Random House. $21.95), Matthiessen’s first novel since “Far Tortuga” in 1975, is a magnificent yarn fueled by a theme that has preoccupied this country’s best authors: the corruption of paradise. A writer who defies easy categorization, the 63-year-old Matthiessen has for the first time put all his farflung interests environmentalism, social activism, spiritual questing-to work on a portrait of a man consumed in equal measure by good and evil.

Because history has little to say about the legendary Watson that isn’t contradictory, he makes a perfect candidate for fiction. He was born in South Carolina in 1855 and arrived in southwest Florida around 1893.

By then, he was already rumored to have killed three people, including his brother-in-law and the Oklahoma outlaw Belle Starr. The mythmaking was underway. By the time he was murdered, Watson was one of the most successful sugar-cane farmers between Tampa and Key West. Everyone liked and admired him, but no one trusted him. Proof was always scant, but people wound up dead when Watson was around.

“Killing Mr. Watson” opens with Watson’s murder. Because his death is foretold, the fated events of his life unfold with a dreadfulness as black as the frock coat he wore in all weather to conceal a Smith & Wesson .38. Matthiessen tells his story through the voices of Watson’s family and neighbors in a series of oral histories, diary entries and old newspaper accounts, all of it fiction. By turns droll, rambunctious, foolish and wise, this collective narration mounts into a carefully orchestrated cacophony of contradictory testimony in which suspicion and mistrust are gradually revealed as the base elements of mystery. As one witness says, “Folks ask, Would I have thronged in with Mr. Watson if I knowed about him what I know today? Well, hell, I don’t know what I know today, and they don’t either.”

On a recent overcast afternoon, Matthiessen talked about his attraction to this enigmatic character. “I’m interested in how a myth is bred out of rumor and gossip and suddenly it becomes a legend,” he said while planting some shrubbery around a pond behind his rambling house on the eastern end of New York’s Long Island. “I’m also interested in the redeeming qualities of a man a lot of people would think is some sort of a monster. We know, but we don’t like to admit, that a man like Mr. Watson could also be a generous man, a humorous man, a good father, a good husband by and large, without taking away for a second from his violent temper and actions.”

It is easy to see how a writer who in his youth kept copperheads for a pets would fall in love with the Thousand Islands region where “Killing Mr. Watson” is set. In southwest Florida, only mosquitoes outnumber the snakes and alligators. Though reared in the manicured landscape of the Northeast, “I’ve always been drawn to back country Florida, ever since I was a little boy,” he says. This hot, trackless territory, where the Everglades break up and crumble into the Gulf of Mexico, has long offered sanctuary to outlaws and misfits, from pirates to drug smugglers. As one character puts it, “You didn’t ask a man hard questions, not in the Ten Thousand Islands, not in them days. Folks will tell you different today, but back then there wasn’t too many in our section that wasn’t kind of unpopular someplace else.”

Matthiessen concedes that his social activism has cost him time as a fiction writer the In 15 years he has produced five books of nonfiction and advocacy journalism, including “The Snow Leopard,” for which he won the National Book Award in 1978. But fiction was his first love. “I don’t make such a distinction as I used to between the two. Now I feel that my fiction and my nonfiction are really saying the same thing.”

On the wall of his studio, he has taped a quote by Albert Camus. citing the obligation “to speak for those who cannot speak for themselves”–whether they be snail darters or Florida Crackers. “I feel about people like that, frontier people, the way I do about anything in this country, any traditionally wild land or wild creatures,” says Matthiessen. “I have an impulse to record stuff before it vanishes.”