Regarding some of the earlier comments that the suspects' apparent post-murder shopping spree indicates a lack of planning and therefore must indicate an accidental death of the victim with no prior malice forethought:
In general, there are two types of criminals in a spectrum. The organized, and the unorganized, or some elements of each. It depends on the individual and how they carry themselves in their everyday life. Just because they may or may not be organized, does not mean that they had no malice forethought to commit a crime. It only goes to show how prepared they are in their commission of the crime, similar to how organized they are in their household, or in their careers and their ability to carry out their work efficiently. In other words, its possible for unorganized perpetrators to wish harm onto a victim, but then wing it as they go along with all aspects during said commission of the crime.
Clearly, judging from the state of the suspects' houses and the profiles of the suspects, these individuals are the unorganized variety. It is possible for these suspects to have malice intent and ill wishes for the victim, but be woefully unprepared in all aspect of their crime, from the murder to the disposal.
So it is not at all strange that an unorganized suspect like that will murder someone. Realize they need to dispose of the body. Then decide they need to buy a saw and other supplies. Only to realize later, even during that earlier shopping trip, that they didn't get all they needed (because they are unorganized). In fact, so unorganized as to make multiple trips for supplies to aid in the disposal process. In fact, my theory is they probably figured out during the disposal process that the circular saw was a bad idea too and promptly didn't use it anymore.
However, just because they decided to buy the circular saw and other items post-murder, it doesn't mean they didn't plan to hurt Becky. I believe they did plan to hurt Becky, but it was their unorganized M.O. that we are observing for why they make so many trips to buy items for the disposal process after the murder. These people are just highly unorganized in every aspect of their life. They will be the same in their career, or their household, as they are in how they carry out murder.
