My BLUF: I'd say that a person with proven past credibility problems (fraud, malicious manipulation, etc.; an "unsavory history") would be more likely to (1) fake an insanity defense; and (2) con people into believing that he is a good guy. That guy would have a pre-disposition to be a good liar; and, yes, a pre-disposition to criminality of the fraud type. That is why evidence of past criminal acts of a similar nature may be allowed into evidence to show a pattern - e.g., a pattern of con artistry. But, most con artists are not murderers.

What could be shaping up here may simply be an evil person (with or without a mental illness component less than insanity), who is in effect a serial killer. That type of person may show few (if any) signs of becoming a serial killer, except in hindsight.

I mentioned that type of person in this post, Measures of Men, although no one commented on him. The grand jury transcript is the source for the stories in Stars & Stripes, Transcript: Suspect killed to become 'real Marine'; and the Orange County Register, Serial killings testimony: Suspect needed to kill. Yup, presumption of innocence, and all that; but it tells why the defendant was indicted.

From the Register:

By LARRY WELBORN / THE ORANGE COUNTY REGISTER

The Iraq war veteran indicted in the murders of four homeless men and two others during a three-month killing spree told an Anaheim detective after his arrest in January that he joined the Marine Corps to be become a killer, according to court documents obtained by The Orange County Register.

But Itzcoatl "Izzy" Ocampo, 23, said he was disappointed when he did not see any combat during a six-month tour of duty in Iraq when he "became a truck driver instead of a killer," according to a transcript of a secret hearing of the Orange County grand jury.
...
(long review of the grand jury transcript in article)
...
Wyatt also told the grand jury that he noticed that as Ocampo was describing each of the murders during the interrogation, his demeanor would change.

"He seemed to get excited when he was talking about the actual kill," Wyatt testified. "So, I asked him if he was aroused by the act of killing."

Ocampo at first questioned the word arousal, according to the transcript, but then added, "my balls felt like they were going to explode, and I knew that I had the killer gene."
The Bales' case, so far, has nothing like this.

Regards

Mike