I'm really leaning towards the fact that he did premeditate this. But - I think that he had to speed up his plan because Stacy told him that day/weekend that she wanted a divorce and wanted him to move out. I think they must have had an argument and he killed her before he was ready to kill her. Then, he's stuck with a house full of kids and a body.
If he had been able to stick with his original plan, he would not have needed help with the body. But because it happened that Sunday, suddenly he has a body on his hands and I bet rigor mortis set in, which made Stacy stiff and he was unable to carry her body by himself.
Stacy had told him about 3 to 4 weeks prior to her disappearance that she wanted a divorce, not just that weekend.
He bought the large chlorine barrel 3 to 4 weeks before her disappearance.
She had been having disagreements and problems with him during this whole period.
Some of Drew's belongings were already packed in the garage.
She had already been checking on places she might be able to rent or move to..Pam Bosco is a realtor..and had been showing her some places.
I think she knew Drew was not moving out so she was checking on her own for a place.
She had been feeling more and more uncomfortable about the situation with Drew..she told friends and family if I go missing..Drew did it. She knew push was coming to shove.
This was 100% premeditated by Drew, in my thoughts, he was preparing from the day she wanted a divorce, and he knew she knew too much to let her just walk out on the marriage.
Drew thoght he had more time then he did to dispose of the body and any other evidence, specifically the diary which more then likely contains references to Kathleen's death and other unlawful acts by Drew..until Cassandra showed up at 2:30 AM AND HE WANTED TO KNOW WHAT SHE WAS DOING..THAT'S WHEN HE FOUND OUT SHE HAD ALREADY REPORTED STACY MISSING.
Drew thought he had more time because he would be the one after a week or so to say she is missing or I heard from her and she isn't coming home.