Centennial, Colorado (CNN) -- To many of his neighbors, Colorado school gunman Karl Pierson was the wholesome boy next door who liked achievement and ran on the cross country team. He even worked on an Eagle Scout project two years ago.
To schoolmates, Pierson was known for his outspoken intelligence that served him well on the debate team. But at times, he acted "weird" and alienated peers with rants about communism and his aggressiveness to win every argument, they said.
One neighbor described him as bright but a social misfit whose peers ridiculed him. His mother had transferred him from another high school because of the mockery and altercations, the neighbor said.
Pierson, 18, opened fire Friday inside Arapahoe High School, where he was a senior. Claire Davis, 17, was wounded in a point-blank shooting, and Pierson, who apparently didn't know Davis, then killed himself in the library, Arapahoe County Sheriff Grayson Robinson told reporters Friday.
"He was a really smart kid. He was intelligent. He knew how to speak; he really did. I don't think I ever won an argument with that kid," junior Daylon Stutz said in the school parking lot on Saturday, when students were allowed to retrieve their cars.
Stutz, an offensive tackle on the football team, had known Pierson since the two shared a human behavior class when Stutz was a freshman and Pierson a sophomore. They worked on a class experiment together in which they went into the community and tried breaking unwritten rules, Stutz said.
"I did think he was a little weird, but I didn't think he was, like, bad weird," Stutz added. "He always kind of talked about how America was a communist country, how the government was, like, trying to take us over and stuff. I don't know, just some weird stuff that I didn't really pay close attention to, but nothing that alarmed me.
"He was definitely kind to me. I never saw him mean to anybody. He wasn't condescending to anyone," he said.
In Friday's shooting, Pierson was armed with a shotgun, a bandolier stocked with ammunition, a machete and three Molotov cocktails, Robinson said. Pierson fired five shots within 1 minute and 20 seconds, he said.
Pierson entered his school looking for the debate team's coach, CNN affiliate KUSA reported, citing Robinson. Pierson was apparently seeking revenge against a faculty member because of a "confrontation or disagreement," the sheriff said.
High school senior Frank Woronoff said he had known Pierson since they were freshmen.
"He was the last person I would expect to shoot up a high school," Woronoff said.
"He was pretty geeky and nerdy but in a charming way, one of the nicest, most humble people I know," he added.
Senior Chris Davis, 18, was among many students Saturday trying to make sense of Pierson's shooting rampage.
"He was a weird kid," Davis said. "He's a self-proclaimed communist, just wears Soviet shirts all the time."
Pierson became easily aggravated, "always liked to be right" and didn't like losing, Davis said.
"It seems realistic, now, that he did it," Davis added.
The home where authorities believe Pierson armed himself is five miles from his school and appeared vacant Saturday. Its front door was sealed and boarded a day after federal agents raided the property and executed search warrants.
A man who declined to be identified in an CNN interview lives a few doors away and said he has known Pierson since he was a boy. In the last few days, the neighbor noticed Pierson driving at excessive speeds throughout their normally quiet, modest middle-class suburb.
The neighbor said Pierson's mother, Barbara, transferred her son to Arapahoe High School from nearby Highland Ranch High School because her son had been subjected to constant ridicule and physical altercations.
"He was socially awkward and just didn't seem to fit into the larger teenage groups, and I think that weighed on him," the neighbor told CNN.
The neighbor said Pierson's parents had been separated for years, and Karl was living with his mother and younger sister.
"While Karl was socially a misfit around kids his age, he was intellectually bright and loved to debate in school," the neighbor said. "If he was disciplined in a debate class, that must have meant everything to him. It may have been trigger point."
Pierson was active in his community, KUSA reported.
He took pride in how he routinely won contests on his speech and debate team, the station reported. He showed off his first place and second place trophies online.
One neighbor described him as a "nice young man," the affiliate said.
In fact, the TV outlet interviewed him seven years ago about the design of a quarter commemorating Colorado.
Pierson submitted questions to the station in 2008 for a show about the Colorado Supreme Court and asked a question at a U.S. Senate debate in 2010, the news outlet said.
CNN's Stan Wilson reported from Colorado, and Michael Martinez wrote and reported from Los Angeles. Ana Cabrera also contributed to this report.