A man convicted of murdering his grandfather is trying to appeal his two life sentences, saying alleged prior sexual abuse at the hands of his grandfather provoked him into the attack. But prosecutors say Casey Collins does not deserve the appeal, in part because he failed to tell his own expert witness about the role he played in his grandfather’s prescription drug scheme.

The Georgia Supreme Court is set to hear the case Wednesday.

The district attorney’s office said the incident began in May of 2013, when Collins, then 30, and his girlfriend, then-23-year-old Sarah Elizabeth Cook, both addicts, woke up craving prescription pills. The two lived in Cook’s grandmother’s Mableton home, and allegedly supported their drug habit through the machinations of his grandfather, Edward Ronald Smith, 78.

Attorneys said Smith would take various people to doctors’ appointments to pick up medications such as Xanax, Oxycodone, Dilaudid and Somas. He would pay for the appointments and prescriptions, but take half the pills to sell.

Cook was one of the people Smith used to pick up drugs, and although Smith would sometimes give them pills on credit, he refused to do so that day, saying Cook already owed him $700, according to briefs filed in the case.

Cook later testified that the two decided then to come up with a plan to murder Smith. She said Collins gave her a pocket knife and told her to stab Smith on his signal. She invited Smith back into his truck and again asked him to front her some pills, but he again refused. That’s when she said Collins gave the signal and she stabbed Smith four times in the chest. When he started fighting back, she said Collins put his belt around Smith’s neck and strangled him to death.

After that, the pair moved Smith’s body to the back of the truck, covered it with a tarp and allegedly went on a drug and shopping spree with Smith’s pills and money, all while Smith’s remains were with them in the truck.

In a plea arrangement, Cook pleaded guilty to armed robbery and aggravated assault and was sentenced to 25 years, with 15 to be served in prison. In exchange for the deal, she agreed to testify against Collins. Following the May 2015 trial, the jury found Collins guilty of malice murder, felony murder, armed robbery, aggravated assault, and concealing the death of another. He was sentenced to two life-without-parole prison sentences plus 10 years.

Now, Collins’ attorneys argue his previous legal representatives failed to attempt to introduce evidence that Collins was abused by his grandparents as a child.

Collins told a forensic psychologist his grandmother had sexually molested him from early childhood to middle-school age. Collins also revealed that his grandfather allowed others to abuse him, but he could not identify them because they wore animal masks. Collins said he was thinking of the abuse when he had the belt around his grandfather’s neck and remembered the pain his grandfather had inflicted on him as a child. The psychologist determined that Collins suffered from Post-Traumatic Stress Disorder.

“Had the jury had the option of finding (Collins) guilty of manslaughter instead of malice or felony murder, and heard evidence that the victim molested (Collins), they could have easily found the Appellant guilty of manslaughter only,” his attorney argued in briefs.

According to the prosecution, Collins was not forthright with this expert witness.

“Collins’s own expert testified that he only came to the conclusion, i.e. that the defendant suffered from PTSD as a result of previous sexual abuse, on the basis of interviews with Collins, in which he was later shown to have been untruthful,” the attorneys write in briefs.

Among the facts prosecutors said Collins concealed was that he participated in his grandfather’s illegal drug operation, that his attack was motivated by his grandfather’s refusal to continue fronting him drugs and that Collins timed the attack for a day his grandfather would have a large amount of cash.

The psychologist testified that knowing these facts and others “would have lessened the likelihood that he would have been able to testify on the defendant’s behalf at trial,” the state argues.

The high court is scheduled to hear arguments in this case during its 10 a.m. session Wednesday.