A jury in Texas finds Eddie Ray Routh guilty of murdering Chris Kyle, the former U.S. Navy SEAL whose autobiography was turned into the hit movie "American Sniper." (Reuters)

Pig assassins. Murderous hybrid pigs. “Seinfeld.” Cologne that smelled like “love and hate.” Air that “smelled like s—.”

These elements were the thrust of a failed insanity defense of killer Eddie Ray Routh, convicted on Tuesday evening of shooting dead “American Sniper” Chris Kyle and Chad Littlefield at a Texas gun range in 2013. And as testimony unspooled inside a courtroom in Stephenville, Tex., and defense attorneys used those words to offer a puzzling and disturbing rationale for why Routh killed a man reputed to be America’s most deadly sniper, there were moments when it looked like the tactic might succeed.

[For Chris Kyle’s killer, Eddie Ray Routh, life in prison may make jail an asylum]

Before the shooting, Routh, a veteran of the Iraq war, was diagnosed with post-traumatic stress disorder (PTSD). Medication for schizophrenia was found inside his wood-framed hovel. Kyle, in a text message to Littlefield the day of their murders, wrote: “This dude is straight up nuts. Watch my 6,” military parlance for “watch my back.” And then, perhaps most compelling of all, there was Texas precedent for a successful insanity defense. In 2006, in another high-profile case, a Texas jury found Andrea Yates, who drowned her five children after experiencing severe postpartum psychosis, not guilty by reason of insanity.

The mother of Chad Littlefield, who was murdered with "American Sniper" Chris Kyle, spoke to members of the media after the men's killer received a sentence of life in prison. (Reuters)

So perhaps, despite the wild admiration in the state of Texas for Kyle, his killer had a shot at convincing the jury he was not guilty by reason of insanity.

But it didn’t work out that way. Instead, the jury took less than three hours to reach a verdict of guilty. And within minutes, the judge handed down a sentence of life in prison without the possibility of parole. So now, as a drama that has absorbed national attention for years draws to its close, a nagging question has been left in its wake. Why, if Routh was so mentally disturbed, did the insanity defense fail? And fail so utterly that a jury needed only a few hours to discard it?

[Marcus Luttrell, Navy SEAL friend of Chris Kyle, warns killer Eddie Ray Routh following verdict]

To understand the answer, it’s necessary to separate the fiction of the insanity defense from its reality. Though mythologized in popular culture and widely believed to be a common defense, the insanity defense is both exceptionally rare and exceptionally difficult to pull off. “When polling graduate students, I often hear guesses that 25-30 percent of all criminal proceedings lead to the insanity defense,” wrote James Hooper in the Saint Louis University Public Law Review. “The truth … is less than half of 1 percent of trials actually lead to insanity of exculpation.”

In general, only the most bizarre cases rise to public notice. And it’s in those cases that the insanity defense comes into play the most — because, the thinking goes, who else but an insane person would do something so disturbing? But “paradoxically, a highly publicized murder … will almost certainly go to trial and also present a steep, uphill battle for an insanity defense,” Hooper continued.

To surmount the odds, an attorney must demonstrate not mental illness but legal insanity. Texas makes clear the distinction between the two in its legal standard: “It is an affirmative defense to prosecution that, at the time of the conduct charged, the actor, as a result of severe mental disease or defect, did not know that his conduct was wrong.”

You can be insane, but if you knew at the time of the crime what you were doing was wrong, you’re cooked.

Even Jeffrey Dahmer, who killed at least 17 young men, some of whom he cannibalized, couldn’t pull it off. His defense more or less followed the same logic as Routh’s: “The thrust of the defendant’s case [was] that Jeffrey Dahmer is crazy — that what he did was so bizarre, so heinous, that he must be nuts,” explained the managing editor of the Milwaukee Journal at the case’s close. But Dahmer was in a tough spot because he went to great lengths to try and hide the bodies of the people he killed, a clear implication that he knew what he had done was wrong.

The exact success rate of the insanity defense isn’t certain. But it’s not very high. In New York, among the nearly 6,000 murder cases to occur in the state between 2003 and 2013, only seven defendants were found not guilty for reasons of mental health, according to the New York Times. In Ohio, by contrast, 15 percent of insanity defenses were successful, a Cleveland State Law Review found. One Texas attorney estimated in an article that the tactic was successful 25 percent of the time in the state.

“The Texas test for insanity is so narrow that it is virtually meaningless,” wrote Brian D. Shannon in a Texas Tech Law Review. “… The Texas insanity defense bears no relationship to modern understanding of serious mental illness.”

For Routh, the tactic was even more fraught. That’s because night he was arrested, he immediately undercut any chance he had. While Routh was questioned by Texas authorities, a ranger asked him, “You know what you did today is wrong, right?”

Routh replied, “Yes, sir.”

Routh could be a schizophrenic, though some experts at trial said they doubted that. He could have PTSD, though, again, some experts said they doubted that. He could have thought Kyle was going to kill him and take his soul, as he claimed. But none of that mattered. Routh knew what he did was wrong when he shot and killed those two men.

And that’s why his defense, no matter what his attorneys tried, was probably never going to work.

Read more:

Mysterious drones loom over Paris for second straight night

Remembering when ‘Parks and Recreation’ was terrible

Botched newspaper exposé of HPV vaccine’s ‘dark side’ reveals dark side of news business

Mysterious East Coast flooding caused by ‘unprecedented’ surge in sea level

State Department official charged with soliciting a juvenile for sex