Here are some of my theories that have some credibility to it -
1. What if the person that broke into the Closs home would have given notes on the layout to JTP? If this was a purely random attack, JTP would haven't have known the layout and would have had a harder time to conceal his steps.
2. Perhaps JTP was trying to break in the same way the prior thief did, but realized that way was blocked, so he shot the door open with the shotgun and accidently shot James. He then knew that with Denise and Jayme as witnesses, he would have been found guilty if LE found him. Then, trying to conceal his steps, shot Denise to death then kidnapped Jayme because he wasn't sure if she would be able to testify against him.
3. Expanding on the fact of the break-in, would it have been possible that James saw JTP and grabbed his gun (This is hunting territory btw) in case he would have came to the front door. JTP shot down the door, and potentially James could have shot JTP in the leg or anywhere (Non-fatal) After he shot James dead, she shoots Denise dead (same reasons as before) and takes the gun because the gun would potentially give his position away, and runs away. He is able to go to the hospital or treat himself, and he is able to survive. He keeps Jayme in the basement because he is afraid that he would get a longer time in jail. He doesn't do any physical harm to Jayme, but inside, Jayme knows she has to escape in case he did.