Sounds to me like some type pf robbery motivated crime. I would think the binding is a sign that whom ever did this needed the victims alive for something i.e. location of valuables, combination to safe, pin number, maybe even was planning to take one of them to the bank to make a withdrawl.
I would guess it is someone who lives in the area, or is related, that heard about them having money or things of value. I would guess that its probibly some shady late teens and/or 20 somethings types and it being rural missouri meth may well be involved. It would not suprizes me if it was a couple of knuckle heads running low on money that had been up for days[on meth] and some one brought up "...I heard so and so has such and such" and they got one of those brilliant meth+awake for days ideas......
very tragic....only slight plus to this would be that it should not be to hard to catch them. These type of ppl are out of their minds ie not very careful, love to run their mouths, and live in a world full of people who are constantly getting in trouble with the law and who are more than willing to sell their own mothers out for a reduced sentance
Note: this is 100% speculation on my part just one possible theory.