Lee Bates, who saw Matt snap his victim's neck in 1997, told CNN's "Anderson Cooper 360" that what Matt did during a 27-hour kidnapping that led to murder was heinous.
"Torture is probably an understatement," Bates told CNN by phone. He said Matt shoved a knife sharpener in his victim's ear at one point then beat and punched William Rickerson in an attempt to get money from the businessman. When Rickerson didn't comply, Matt broke his neck then dismembered the body.
Bates, who drove the car in which Rickerson was tied up, testified in 2008 against Matt and is concerned the escapee might seek revenge. He said he was scared of Matt during their crimes and was scared of him now.
The one-time accomplice told CNN that he knows Matt put a lot of thought into planning his latest jailbreak.
"He is a very cunning and dangerous individual," said Bates, who was convicted 17 years ago of second-degree murder. He served 15 years.
Matt and another inmate from the Clinton Correctional Facility in northern New York, David Sweat, now are trying to figure out their next move, following a weekend prison escape.
The escape -- which involved power tools, navigating a maze of pipes through and out of the prison -- seems so unexplainable at the outset that even cliches fail to describe it. A daring escape? Brazen? Perplexing?
How did they do it? And what's next?
All those things, kind of. The pair used decoys to make it look like they were sleeping while they tunneled their way out.
Both men, Matt and Sweat, are convicted murderers. And the biggest question beside "How did they do it?" is "What could they have planned next?"
As the pair's back stories emerged, more was known about Matt than Sweat.
Sweat, 35, was serving a life sentence without parole in the killing of Kevin Tarsia, a sheriff's deputy, in 2002.
Sweat and another man robbed a gun store, and when confronted by Tarsia, responded by fatally shooting him.
The slain deputy's siblings on Monday said they couldn't understand how their brother's killer escaped.
How did no one hear the power tools Sweat and Matt used to make a hole? Sharon Tarsia asked.
Steven Tarsia said he was surprised to learn the escapees had adjoining cells in an "honor block" reserved for inmates with the best behavior.
"I was under the assumption that they would never get anything. Their lives would be miserable, and awful. That's what we wanted," Steven Tarsia said.
Matt considered a violent person
Gabriel DiBernardo, who was the lead detective in the investigation into Rickerson's death in 1997, said he was not completely surprised about the jailbreak.
Matt had escaped from jail once before, in 1986. According to the Buffalo News, he was on the lam for four days before being recaptured.
"He has a history," DiBernardo said. "He broke out of jail before. He is a cunning individual, no question about it, and a vicious individual."
Bates said it's possible Matt got someone who worked at the prison to help the escapees, who had adjoining cells.
"He can make friends easy. He's a master manipulator," Bates said.
DiBernardo said he wouldn't be surprised if Matt had more to do with the escape than Sweat.
According to a Buffalo News report from before Matt's trial, he had been in Mexico, where he allegedly killed another man.
He was accused of killing a man in Matamoros, Mexico, outside a bar and served nine years in a Mexican jail. During his trial for murder of Rickerson, Matt's attorney said his client never had a trial in Mexico and "suffered indignities that we wouldn't impose on stray animals in our country."
Matt worked for Rickerson and was fired for poor performance just weeks before Rickerson's murder, according to coverage of the trial.
Reports from the trial included another telling anecdote: During the trial, Matt wore electrodes on his ankles or waist that could be triggered by a guard if needed while inside the courtroom.
The Buffalo News noted yet another twist: The day after Matt's conviction, a police detective and former friend of Matt received a note from the inmate.
The note accused the detective of having lied during the trial, and said, "You also make it very clear that we are not friends. I'll remember both ..."
The ellipsis left the note open-ended.
During the Rickerson trial, the issue of Matt's previous jailbreak came up.
His defense attorney explained it this way at the time: "He told me, 'I was 22 then. I climbed a fence. I couldn't do that now.'"