The Barrels.
Why barrels? Obviously to conceal them. Particularly during transporting them. This also implies that a truck was involved, as I doubt a 55 gallon drum would fit in a car trunk.
But, short of incinerating, or sinking them to the bottom of the ocean - burial is the next best choice. But they didn't bury them. Could it be because they were killed in the winter, and the ground was too frozen?
Also - barrels were probably easier to get back then. They are well regulated now. But they would have been worth something... Could they have been reported stolen? If not because of their own value, but because someone broke in someplace to get them?
The barrels and the actual crime:
The person either had the barrels before or after these people were killed. If he had them before, it implies that either this crime was pre mediated, by virtue of the fact he had them already - or he had ready access to them, after he killed them, in cold blood, and knew he could get them.
If he didn't have them - I think this means they were killed inside a private residence. Where they would go unnoticed while he went and found the barrels.
I'm not sure how many premeditated murders are committed by bashing people's heads in, but I'm guessing not a big percentage.
Also, putting them in barrels that can be rolled could make transporting them, and rolling them into the woods, a one man job. But it couldn't have been easy.
Also, if the barrels were filled with whatever material they were designed to hold like solvent, oil, grease, fertilizer that would make them even more valuable, and more likely to be reported stolen.
Is it possible to determine what these barrels were used for? That could help narrow down where they were taken from, and perhaps provide new clues.
Lastly, do we know if there was any clothing found with the victims?
Sorry to ramble. This story really bothers me, so unbelievably cruel.