STEPHENVILLE, Texas — The former Marine convicted of killing “American Sniper” author Chris Kyle and another man was hospitalized multiple times for psychiatric treatment and was prescribed medication to treat schizophrenia. He spoke of pig-human hybrids and the apocalypse and was described by Kyle himself as “straight-up nuts.”

But jurors found that the insanity defense for Eddie Ray Routh failed to meet the legal threshold: a mental illness so severe he didn’t know right from wrong.

“The insanity defense is very rare, and it’s even rarer that a defendant wins it,” said George Dix, a law professor at the University of Texas at Austin.

He said when a brutal crime is committed it’s difficult to convince a jury the person accused doesn’t “deserve the condemnation that comes from a finding of guilty.” He added, “And here, we’ve got him causing the death of an American hero.”

Kyle, a former Navy SEAL sniper, volunteered with veterans facing mental health problems after he left the military. A blockbuster film based on his memoir about his four tours in Iraq contributed to intense interest in the case.

Legal experts say a defense attorney’s task to convince a jury that a client is legally insane is even more difficult in cases like that of Routh, who confessed to killing the men, apologized to the family and fled from police.

“If someone is admitting that they committed the murder, it’s a pretty tough burden to get a jury to say, ‘Let’s excuse him anyway,’ ” said Dallas defense attorney Michael Snipes.

Kyle and his friend, Chad Littlefield, were killed after taking Routh to a shooting range on Feb. 2, 2013. Routh’s mother had asked Kyle to help her son, who she said suffered from post-traumatic stress disorder.

Routh’s attorneys said he was having a psychotic episode at the time of the shootings, and noted that Kyle described Routh as “straight-up nuts” in a text message to Littlefield as they drove to the range.