Four years ago, two young men went into the marshy woods along Brook Street in Islip after dark. One came out alive and one was found three days later after having been beaten, choked and buried alive in the muck.

At the end of a trial that began Monday, jurors will decide whether the man who walked out was justified in killing the other or if he is guilty of second-degree murder.

There is no dispute that Thomas Liming, now 23, of Islip, killed his former Islip High School friend, Kyle Underhill, 18, on Nov. 16, 2011, in those woods. The question that the trial, expected to last past Thanksgiving before state Supreme Court Justice Mark Cohen, will answer is whether Liming had a legally valid reason to do it.

Assistant District Attorney Kathleen Kearon told jurors in her opening statement in Riverhead that the extraordinarily brutal killing and the "massive cover-up" that followed should answer that question.

Underhill was beaten severely in the head and torso, choked, had two sticks rammed down his throat and was buried while still alive -- "sheer, massive overkill," Kearon said.

"Do not get distracted by illogic," she said. "This is not a factually complicated case."

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But it may be an emotionally complicated one, defense attorney Joseph Corozzo of Manhattan said. His client and Underhill were good friends until shortly before graduation, when they had a falling out.

Underhill was known in school "to be somewhat dangerous," Corozzo said. "He's always wearing dark clothing. He makes threats to shoot up the school, blow up the school. . . . He exhibited hatred toward other students in that school."

After the friendship fell apart, it was Underhill who threatened Liming repeatedly, Corozzo said. The day he died, Corozzo said, a co-worker at the bakery where he worked overheard Underhill muttering: "I'm going to kill him. I'm going to kill this kid. I'm going to jail for murder tonight."

And then Underhill packed bleach and rubber gloves and brought them to the woods with Liming, Corozzo said.

"There was evil in the Brook Street woods that night, but it wasn't Tom Liming, and the people can't prove it was Tom Liming," Corozzo said.

The interaction between the two that day began with a phone call, and Liming showed up shortly afterward at the bakery where Underhill was working. There was a brief but intense confrontation there, both lawyers said, but Liming then helped Underhill finish up his work. They left at 6 p.m.

By 9:15 p.m., Kearon said, Liming was at home starting his cover-up. His mother called 911 to say her son had been assaulted, but when a police officer showed up he saw no injuries. Kearon said that in the middle of the interview, Liming's mother talked to an attorney and she abruptly told the officer he was no longer needed.

Corozzo said his client shouldn't be penalized for realizing he should protect his rights.

Police Officer Robert Daniels testified about finding Underhill's body on the third day of searching. He said he saw what he thought was a birch branch protruding from beneath a plywood board, but soon realized it was the body. After the board was removed, the body was barely visible in a hole filled with muddy water.