Your posting reminds me of something I had been wondering about a few weeks back. It seems that one of the mysteries of problems with the "crime of passion" theory is the killing of Dr. Kelly and Mr. Niederbrock. It occurs to me that we don't actually know if Dr. Kelly actually let Sam stay at the house on their return to Virginia. If Emma and Melanie were not looking forward to riding back with Sam from STFW, imagine how they felt knowing he going to be staying at Emma's house for another week.
WARNING TOTAL SPECULATION AHEAD
One of the things that has been bugging me about the basic scenario as we have discussed it is that the weapons are tools that were probably stored in a shed outside the house. We don't know this for sure of course, but if true he had to exit the house to locate and retrieve the weapons and then quietly re-enter the house.
Here's an alternative: perhaps the girls lobbied mom to kick him out and maybe she did it. Perhaps this was discussed on the way back in the car with dad present to give it a little extra force for Sam. This theory would explain the murders of Dr. Kelly and Emma's dad IMO.
Now he's stuck in Farmville with nowhere to stay and not enough money to get a hotel room or get home. He can't call his parents to get more money. Perhaps he lurks around the yard and house at night entering the shed and finding the weapons there. Possibly he also had Emma's keys somehow which explains why they were found outside the house...
Anyway, as I said, all of this is pure speculation. But if true, it does help to explain the violent murders of the parents in addition to the girls.