Set her soul free - I think that this is what he originally intended to happen, too, and something went awry that caused him to change his story and place himself in the home when the crime was committed. My thoughts are that perhaps the gun jammed and when he realized Melinda was still alive, he went into a panic and was worried she might live and be able to talk to the police, so he changed his story.
It appears to me that he very carefully planned Melinda's murder, from setting up the life insurance (or so he thought) to making police reports on the "stalker", to trying to establish he had been staying at his brother's house in recent weeks, to leaving behind no forensic evidence that he was the shooter. I think a lot of effort and planning went into this and it blew up in his face when whatever went awry with the plan, went awry.
I also think if the original plan had worked, you would have had to live through the additional and horrific trauma of finding your dear friend. And possibly the baby, too. I also think that getting a conviction of SL would have been more difficult if he had been able to say he was elsewhere (perhaps staying at his brother's house) when the crime was committed. I'm not sure if anyone from the jury has spoken, but my guess is what left them with no reasonable doubts about his guilt had a lot to do with the fact that his statements about being choked out and dragged into the baby's room did not match the evidence at the scene.
It is very likely he will never say what really happened, leaving everyone to speculate about certain details. But one thing we probably don't need to speculate about is how truly horrible life will be for him in prison. Even before he murdered Melinda, SL flagrantly broke rules, even when he had to know that breaking certain rules would bring about very serious consequences. He lost a spouse, a job, and was in trouble with the court over failure to honor his child support obligations. In prison, he will be required to follow rules every single minute of every single day. He is going in as a convicted murderer and as such, no one is going to coddle him. He breaks the rules in there, he is going to face some very serious consequences.