IMO, the reason you are having difficulty understanding how these men could plan and carry out their perverted evil acts, is that you seem to be looking at them using the operative word "normal". There is nothing about these two which we can consider "normal". They are pure EVIL, warped and demented.
Thankfully, this persona usually comes along with a terribly inflated ego which makes them believe that their cunning is intellectually superior, which in this case, the Vermont State Police and the Feds have gloriously proved, beyond a shadow of a doubt, to be only a figment of their own imaginations.