With the Idaho case, I’ve spent way too much time trying to think of how you would avoid tying yourself to a vehicle. Career criminals just steal cars (then burn them), which seems to work, but if you don’t have a criminal background, you probably just increase your odds of getting caught if you attempt that.
I recall from the Idaho case that the search for matching vehicles only reached to neighboring states. So, one could get a long term rental (like, one month), from somewhere at least 1,000 miles away. 2 day drive, take your time and see some sights. Somewhere in the middle, steal a license plate. Ideally from a car with the same make and model, and from a location without cameras. Swap the plates (somewhere isolated). Swap back somewhere else isolated along the way back. Benefit to keeping the car for a month is you’ll be less likely to be suspicious for having a large number of miles on the car.
It seems surprising to me that someone with a surgeon’s background and money wouldn’t go to those steps, since it seems simple enough to me, but it may have been a more spur of the moment crime with limited planning. Which I do think makes sense. Months of anger and obsession build up, and then something over the holidays triggered him to snap.
Maybe a couple days of planning. Not long enough to do it well or, more importantly, to come to your senses and not do it at all. My guess is between getting the impulse to actually kill, and following through on the murders, he had no distractions (didn’t go into work; didn’t get together with family, etc.). But that said, the holidays probably aren’t a coincidence. Getting together with family, going back to your hometown (?), etc. can all be emotional.
From following true crime, it has always seemed to me like a disproportionate number of murders happen in December.