Devin Baham, on trial for second-degree murder, arson and obstruction of justice in the 2012 stabbing death of Ashley King, was the driving force behind robbing and killing the Slidell woman and burning her apartment, the prosecution said Tuesday as the trial opened.

Assistant District Attorney Julie Knight laid out the state’s case, in which Andrew Sumner and Katelyn Marie Lusich, who previously agreed to plea deals, will be key witnesses against Baham.

Knight told jurors that they will hear testimony that Sumner tried to call off the robbery — or at least make sure it did not turn violent — and that he hid in a kitchen alcove listening to King’s dying screams as Baham stabbed her and then embraced Sumner, telling him that everything would be all right.

But Baham’s lawyers, Martin Regan Jr. and Ravi Shah, made it clear they plan to put the credibility of the prosecution’s star witnesses on trial. Shah, who made the opening argument for the defense, called Sumner and Lusich liars and cowards. “They did what cowards always do: blame somebody else,’’ Shah said.

He said prosecutors would be unable to present any physical evidence tying his client to the homicide.

He said Baham, now 24, was nowhere near King’s Bayou Lane apartment on Feb. 22, 2012, when she was killed, or the following day, when the apartment was set ablaze. Shah said it was Sumner who bought the murder weapon, a fishing knife, and it was Sumner who knew King and had bought pills from her in the past.

His client did make a mistake, Shah said, but that mistake was to befriend Sumner, a young man from an affluent family who he said suffered from such a ferocious drug addiction that he could not wait to buy drugs from King but decided instead to rob her.

Sumner pleaded guilty to manslaughter, aggravated arson and obstruction of justice and will be sentenced after he testifies in Baham’s trial.

Lusich, who was a pregnant 17-year-old at the time of King’s slaying, pleaded guilty to obstruction of justice in September. The counts of aggravated arson and second-degree murder that she faced will be dropped after she testifies, according to a plea agreement.

Neither of them took the stand Tuesday. Instead, much of the testimony involved a reconstruction of the arson scene where King’s badly burned body was found early on Feb. 23.

Jason Johnston, an arson investigator with the Louisiana Fire Marshal’s Office, described the apartment and the condition of King’s body in sometimes gruesome detail as jurors viewed pictures of the crime scene.

The most gripping testimony of the day came when King’s sister and father took the stand to talk about the young woman who had moved to Slidell from Louisville, Kentucky, to be closer to her father and who had met a man she planned to marry.

Brittany King called Ashley her “best friend’’ and described her sister as outgoing, kind and trusting of others. “She never knew a stranger. No matter what kind of day you were having, she could always put a smile on your face,’’ she said.

Brittany King smiled as the prosecution put up family pictures but grew tearful when she was asked about her last communication with her sister. Ashley had called while she was at work and she had not taken the call, she said in a choked voice.

Ashley had recently sold her BMW, with plans to use the money to start a spray-tanning business like one Brittany had launched in Louisville, her sister said.

But the $10,000 in cash she received for the car made her father, Joe King, nervous. He recounted a phone conversation with his daughter when he learned she was at the Fair Grounds and had the money on her.

“They’ll rob you of $10; they’ll kill you for $10,’’ King, who is a thoroughbred trainer, said he told his oldest daughter.

He said that several nights before her death, she had heard someone trying to break into her apartment and had hidden in the closet with a butcher knife.

Ashley suffered from back pain that had been made worse following a car accident, her father said. The morning of the day she was killed, he had picked up her prescription for oxycodone and then taken her to Wal-Mart to buy a table for her tanning business.

He had to leave for Lafayette, he said, but had promised Ashley to talk to her by phone at 6 p.m. When he couldn’t reach her, he became fearful, he said, describing efforts by friends and family to reach her.

“She was 32 years old. She had a mind of her own. She had lived all over the world,’’ her father said.

Erin Marks, a friend of the murdered woman, also testified about her last phone conversation with her, midafternoon on the day of her death. She said King told her “Andrew’’ was coming over to help her put a desk together.

Regan questioned Marks closely about that conversation, pressing for details such as Andrew’s last name and his relationship to the deceased. But Marks said she knew nothing more about it.

Regan also questioned Marks about her friend’s drug activity. Marks said she had never seen King sell drugs but that King had admitted selling pills. “I only knew she had done it, not how often,’’ she said.

Testimony will resume Wednesday.

Follow Sara Pagones on Twitter, @spagonesadvocat.