What pushes them over the precipice and into the abyss of sociopathy in which they reside is that they insist that their parents do not have the right to make the same sort of claim on their finances; as we've seen, the duty that they feel that they owe their parents is to warehouse them with inept caregivers. Given that we're dealing with the same brand of self-righteous complacency in both cases, it doesn't take a genius to realize that a feeling of deprivation is what motivates them.
You and I might ask what possible privation could either of them experienced to make them almost bug-like in their petty avarice. A rational person would have seen two sets of parents doing their damnedest to make sure their children went farther in life than they did; the problem is that John and Elly aren't rational in the least. We know from all the ranting Elly does to Phil that she felt that her loving, tidy little home was a bleak horror in which she endured incalculable want because, as Marian might put it, her eyes were and are bigger than her stomach. This irrational envy whipped up by her immaturity, idiocy and lack of empathy is one of the things she shares with John; his fear of having to answer to a boss like the father who never provided as well as John thought he would not only explains his need to surround himself with the toys he wanted, it also explains the need for obedience.
Not, of course, that they'd admit to themselves that they hate their parents for being poor; that would be bad form and also expose them to public ridicule.