It's a common refrain heard from neighbors and friends after someone they know is accused in a mass shooting or other horrific crime: "He was such a nice guy! He was so quiet -- I can't believe he'd do something like this!"

Even serial rapist and killer Ted Bundy, who admitted to killing more than 36 women in the 1970s, was roundly described as charming.

And now accused cop killer Christopher Dorner seems to belong to the group of people who, if California authorities are correct, duped most everyone around them when it came to their sinister side.

"Chris was a guy who was approachable, had a great sense of humor, fun to be around, intelligent, good conversationalist, the kind of guy most people would want to hang out with and want to spend time with," said his friend and former Southern Utah University classmate James Usera.

"He was really, in my experience, a pretty terrific individual for whom I had a great deal of respect."

A former next-door neighbor also had a good impression of the accused killer, saying Dorner was "a nice, friendly guy, easy to approach."

Approachable, funny, terrific -- hardly the description you'd expect of a man who commanded a $1 million bounty after he promised a campaign of "asymmetric warfare" on the Los Angeles Police Department, his onetime employer.

Police say he followed through, killing four people, two of them police officers. He wounded four others. Authorities are now working to determine whether a body found in burned-out cabin near Angelus Oaks in Southern California belongs to the 33-year-old man who allegedly terrorized the force.

While Dorner gave off a certain impression to many, some who knew him best, like ex-girlfriend Ariana Williams, had seen another side of him.

Their relationship started when Dorner was a rookie cop in a tough, stress-filled beat, and ended with Williams posting a warning on a website for other women to avoid dating Dorner.

Williams was in shock at what unfolded this week, but was not completely surprised that Dorner was capable of acting out like that.

"When you see someone's behavior, and their manner of stress-induced behavior, coupled with how their personality already is, I think it makes it easier to understand why something like this could possibly happen," she told CNN Thursday.

He always kept weapons out in the open and nearby when he would visit her home, and lacked an even-keeled personality, Williams said.

She was not scared of Dorner, she said, but she had a persistent feeling that something wasn't right about him.

After their relationship ended, she felt compelled to go on the Internet and post a warning to other women who may date him in the future.

"Just in the general -- the fluctuation of his behavior, the swinging from the highs to the lows, the extreme nature of his personality," led her to say something, Williams said.

The killing rampage began Feb. 3 after Dorner claimed he was railroaded out of the department after filing a brutality report against another officer. Dorner accused the LAPD of a longstanding culture of racism and misconduct that had worsened since the 1991 Rodney King beating.

In his purported manifesto, he levies several accusations against his former employer and, in one instance, demonstrates his willingness to exact violence on his co-workers.

After confronting an officer he claims used the n-word, Dorner claims to have grabbed the officer by the throat "and squeezed," but he writes that he regrets not doing more.

"What I should have done was put a Winchester Ranger SXT 9mm 147 grain bullet in his skull," the manifesto states.