As he built a successful real estate firm, Todd Kohlhepp’s behavior struck some as odd: watching pornographic videos during work, making macabre jokes in marketing literature and openly discussing that he was a sex offender.

But he was also an award-winning agent described as a hard worker and good boss.

All the while, authorities say, he was hiding a grisly secret that included seven killings over 13 years. He gunned down four people at a motorcycle shop in 2003 and within the last year killed three more, authorities said. The crimes were uncovered earlier this month when investigators rescued a woman chained up in a 30-foot-long (nine-metre) storage container on his property.

“He was the kind of type-double-A, hair-on-fire kind of guy ... You felt that if he wanted to take you out, he could take you out easily,” said Lawrence Shorts, a mortgage banker who worked on transactions with Kohlhepp.

You used to walk in his office and he had porno going on Lawrence Shorts

Kohlhepp made ominous comments about having trouble sleeping and how he would “know where people live”, Shorts said. A neighbour, Scott Waldrop, said Kohlhepp bragged about chasing people off his rural property with an arsenal including guns with silencers and night-vision equipment.

Kohlhepp moved to South Carolina sometime after the 2001 completion of his 15-year prison sentence for raping another teenager at gunpoint in Arizona. He was in his early 30s and began studying at Greenville Technical College in 2003. While attending classes that November, he killed four employees at the Superbike Motorsports store in Chesnee, authorities said.

Those slayings went unsolved for 13 years. When Kohlhepp was arrested last week, the sheriff said he confessed to the bike shop shootings and pointed investigators to three shallow graves on his land.

Detectives notifying victims’ families of Kohlhepp’s confession said he was an angry customer at the shop. Motives aren’t clear for the more recent killings and the woman’s kidnapping.

Todd Kohlhepp’s home is searched by Spartanburg County Sheriff’s deputies. Photograph: John Byrum/AP

In 2004, Kohlhepp transferred to the University of South Carolina-Upstate and ranked near the top of his class the next year, said college spokeswoman Tammy Whaley. He finished his business degree in 2007, then distinguished himself in real estate. A 2008 news article said he was Weichert Realtors’ top-selling rookie agent for a region spanning parts of both Carolinas.

Next, he went into business for himself, registering Todd Kohlhepp & Associates in 2009. The firm’s website listed over a dozen agents, but also an unusual sales pitch. Describing the team that developed the firm’s marketing, it said: “We threatened not to feed them if it didn’t work. It’s amazing the motivation you can get after day three!”

Still, Kohlhepp developed a hard-working reputation.

“Todd did his job well and we never had any issues or saw any flags in the years we worked with him,” the Velocity Design Group owner, Danielle Cuddie, said in an email.

An agent at Kohlhepp’s firm, Cherry Laurens, said they met a decade ago and were study partners at USC-Upstate. She said the accusations surprised her.

“Everybody loses their temper, but as far as out of character for what would be normal? No,” she said.

Shorts, the banker, said Kohlhepp appeared successful, driving expensive cars. Records show he owned two BMWs and a motorcycle. Kohlhepp worked from home, and Shorts came over occasionally to exchange documents.

“You used to walk in his office and he had porno going on,” he said. “I can remember walking in there two or three times and there was pornography on his dad-gummed laptop. Realtors would walk in and he had it on there too.”

Shorts distanced himself in recent years, saying Kohlhepp would make statements such as: “You know I don’t sleep much at night and get up at three o’clock in the morning and I kind of know where people live.”

Kohlhepp appeared to be a loner. His neighbourhood was close-knit, but neighbours said he never attended its occasional get-togethers. Kohlhepp’s sex-offender status was common knowledge, though Kohlhepp claimed the charges were trumped up after a joyride with another teenager upset her father, acquaintances said.

But authorities described something far worse: Kohlhepp was 15 when he tied up a 14-year-old at gunpoint and raped her in Arizona in 1986. Court documents say he pleaded guilty to kidnapping and was sentenced in adult court.

Kohlhepp had psychological problems from a young age, according to records obtained by WHNS-TV in South Carolina. An Arizona judge wrote in 1987 that he was bright but “emotionally dangerous” and preoccupied with sex. A 1987 pre-sentencing report says a neighbour described Kohlhepp as a “devil on a chain”.

He had moved in with his father in Arizona around age 12 after growing up in South Carolina and Georgia. His parents divorced when he was an infant. Now his mother is defending him, telling CBS: “Todd is not a monster.”

Meagan Leigh McCraw Coxie and Johnny Joe Coxie, a husband and wife who have been identified as the second and third bodies found at the South Carolina property of suspected killer Todd Kohlhepp. Photograph: Spartanburg County Coroner

“He wasn’t doing it for enjoyment,” Regina Tague said of the South Carolina cases. “He was doing it because he was mad and he was hurt.”

On Facebook, he frequently described trimming trees and moving boulders on the 95-acre (38-hectare) property near Woodruff, about 10 miles (16 kilometres) from his primary home in Moore. In September, he wrote that buying land was a way to stay out of trouble: “You won’t have any time to misbehave no mo ... too sore and tired.”

But that sprawling property was where authorities said they rescued the chained woman and began unraveling the killings. Investigators say they unearthed three bodies there. Authorities also confiscated assault rifles and handguns. It’s not clear how Kohlhepp obtained them; South Carolina law prohibits violent felons from owning firearms.

Waldrop, the neighbour, met Kohlhepp soon after he bought the property in 2014. To keep people out, Kohlhepp installed deer cameras, bear traps and a chain-link fence.

“To me, he didn’t seem like no threat,” Waldrop said. “We’d just shoot the bull ... I didn’t know all this stuff. Nobody knew.”