'American Sniper' Trial: Sister Says Eddie Routh 'Traded His Soul for a Pickup'

— -- Accused killer Eddie Ray Routh seemed confused when he arrived at his sister's home a few hours after allegedly shooting Chris Kyle and Chad Littlefield, Routh's sister said today.

"He was talking about pigs sucking his soul," Laura Blevins testified during the trial.

Routh's sister and her husband Gaines Blevins were two of the witnesses the defense called to testify this morning in the double murder case. Kyle was a former Navy SEAL and the focus of the book and the movie "American Sniper."

But the clear star of the defense case so far has been Jennifer Weed, Routh's ex-girlfriend. She has given the jury the most powerful testimony yet of Routh's history of mental illness.

They spent the night before the murders together, she said, during which time Routh told her the government was listening to him. Routh asked her to not say anything out loud and write down anything she had to say so "they" wouldn't hear it, she said.

And then, out of nowhere, Routh asked her to marry him, Weed testified. On the morning of the crime, they fought about his tobacco use and she left, she said.

Laura and Gaines Blevins called 911 after Routh came to their Texas home on Feb. 2, 2013, and admitted killing the famed sniper and his friend at a gun range.

Laura Blevins said that she knew something was awry when her brother pulled up in a truck rather than his normal car, a Volkswagen Beetle that he had gotten painted to look like a ladybug.

The car he was driving on the evening of Feb. 2 was Kyle's SUV that he allegedly stole while fleeing from the scene.

"He said he traded his soul for a pickup," Laura Blevins said on the stand. "The person who came to my house is not who I know as my brother."

Gaines Blevins said that Routh "said he took two souls before they could take his." That claim is the same one Routh made again later that night to Texas Ranger Danny Briley, whose interrogation video was showed in court on Monday.

Today marks the first full day of the defense's case, and while they have never disputed that Routh killed Kyle and Littlefield, they are arguing that Routh was insane at the time of the shooting.

Under Texas law, that means that they will have to prove that Routh did not know what he was doing was wrong. He faces life in prison if found guilty.

Weed also told jurors about one of Routh's "episodes" two weeks before the murders. Routh grabbed some decorative ninja swords and held his girlfriend and her roommate hostage in their Dallas apartment, she told the court. He told them they couldn't leave because people were looking for them and the only safe place was the apartment, she recalled, noting that police finally came and Routh was taken to a mental hospital.

This is a shortened court day. The girlfriend is still on the stand and may be the last witness for today.

Get real-time updates as this story unfolds. To start, just "star" this story in ABC News' phone app. Download ABC News for iPhone here or ABC News for Android here.