The roommate, 26-year-old Jaime Eheler, also said when Roberts left the house they shared, "he had a weird look on his face."
Hours later three people were dead, including Roberts, who shot himself after the Tuesday rampage.
Although officials have not yet revealed a motive, a clearer picture of Roberts is emerging. His mother died of cancer when he was young, he was thrown out of his aunt's house as a teenager and had lived with Eheler's family for a while, and he had told friends of plans to live in Hawaii for a year.
"I never saw this young man raise his voice," Eheler said, sobbing. "I've seen him sad, I've seen him hurt. I've never seen him mad."
The Clackamas County sheriff's office said Roberts had several fully loaded magazines when he arrived at the mall Tuesday as thousands did their Christmas shopping. Roberts parked his 1996 Volkswagen Jetta in front of the second-floor entrance to Macy's and walked through the store into the mall and began firing randomly in the food court, authorities said.
He fatally shot Steven Mathew Forsyth, 45, and Cindy Ann Yuille, 54, the sheriff said. Kristina Shevchenko, 15, was wounded and in serious condition Thursday.
Eheler said Roberts had recently quit his job at a gyro shop in Portland, sold all of his belongings, and put his car up for sale on Craigslist in advance of his planned move to Hawaii.
She said that his decision to move to the islands caught her by surprise, but only a little bit. She said he told her he planned to live off the interest of an inheritance that he had invested, and that he planned to return on Oct. 25, 2013.
"He's adventurous," she said. "That's who he is."
Roberts' ex-girlfriend, Hannah Patricia Sansburn, told ABC World News with Diane Sawyer that he was supposed to catch a flight Saturday, but told her he got drunk and had missed it.
The owner of Big Bertha's, where Roberts most recently worked, threw a going away party for him last week.
"His nickname there at the shop was 'The Kid,'" said Thomas Illk, father of Tommy Illk, who owns the shop. "Tommy is just devastated. He was like a little brother to him."
Police say Roberts had stolen an AR-15 rifle from someone he knew. Authorities said that after the shootings, Roberts fled along a mall corridor and into a back hallway, down stairs and into a corner where police found him dead from an apparent self-inflicted gunshot.
The shooting confounds Eheler. She said she knew that Roberts had gone target practice shooting in the past, but she said there were no guns in her house, and she'd never seen him with one.
"I can't wrap my mind around it, I can't understand," she said. "I sit here and think, and I think, and I think, and I can't come up with one reason why he would do this."
Eheler said that she later learned that he had stopped at her brother Tyler's home on the way to the mall. Tyler Eheler and Roberts had been best friends since high school, she said.
"He came in and hung out for a minute and told him that he had to go and that he didn't want to," she said, and added that he gave her brother a bracelet that he always wore, and hugged him. "He told him he was just going somewhere south, somewhere warm and not to tell me or my boyfriend that he had left until the next day."
Eheler said her brother was devastated and was not granting interviews.
Another friend who said she had known Roberts since middle school, Shania Riley, described him as a "very compassionate and very caring person." Roberts could also be "mysterious" and "kind of a troublemaker."
"Like being young, skipping class, smoking pot out back - teenager stuff," Riley said. Everything seemed fine when she saw him at a bar last week, she said.
The only odd moment Riley could recall came about six months ago when she and her boyfriend were at Roberts' apartment "and he was pulling out guns and showing them to us."
She recalled seeing two handguns and what might have been a .22-caliber rifle.
Still, Riley didn't find it alarming. Roberts said he used them for target practice out in the hills, she recalled.
Benjamin Eshbach said he used to play chess with Roberts when he was general manager of a music theater next door to the gyro shop.
"He was very good," Eshbach said, noting they played on the community chess board in the shop.
Eshbach said Roberts had bought a pistol about a year ago, and used to shoot it in the woods when he went camping.
"A lot of people are thinking he was fixated on things like that. That couldn't be further from the truth," Eshbach said.
Contributing to this report were Associated Press writers Tim Fought, Jonathan J. Cooper, Nigel Duara, and Sarah Skidmore in Portland, and Jeff Barnard in Grants Pass, Ore., along with researcher Rhonda Shafner.