Curiositycat...............I think Kathleen's murder was premeditated too. With Stacy, I think DP may have thought about killing her, especially if he knew she was planning on a divorce and if she knew that he had killed Kathleen. But, I think that even if he had thought about it, he had not thought out all the details yet.
I think Stacy's death happened in a fit of rage because of the circumstances of her death. If premeditated, he wouldn't have chosen a Sunday morning with all the children in the house, and neighbors home. He would have chosen a weekday when the two oldest were in school and when many of his neighbors would be at work. I don't think he would have killed her in the house either. He would have made an excuse to get Stacy out of the house alone.