Wildblood states the basic idea:
It would be my contention that just as there is a divine plan for mankind so there is also a demonic plan... Their aim is to absorb the energy from human souls for their own benefit and use. They have cut themselves off from the source of life and need life energy to maintain their existence. So they seek to corrupt souls.
Pure spite just doesn't seem like enough of a reason for the devil to keep working tirelessly, millennium after millennium, for something that brings no benefit to him personally. It seems plausible that the devils must in some way need human damnation, that they must be soul-predators, vampires.
This would be an extremely valuable addition to understanding positive demonic motivation. Perhaps we could look at our own evil acts and those of others - in this mortal life - to see whether an assumption of some kind of life-energy vampirism fits what we have observed. I think it does.
Of course we can't actually perceive or physically measure what is going on - in terms of energy transfer - but it certainly seems as if this can happen.
Furthermore, from what I have read of the dark side of the ritual magical societies of the late 19th, early 20th centuries (including what found its way into Charles Williams novels - who knew this world from experience) energy-vampirism seems to be almost exactly what dark ritual magicians were trying to do - and almost certainly sometimes did.
This certainly supports the idea that it is what demons do; as does what I have read in (for example) David Icke and elsewhere about the kind of rituals that the Global Establishment engages in. A great advantage of knowing this is that it means we can get away from discussing 'demonic psychology' - what they like, what they want etc - which all sounds (as you say) too mushy and superficial to lead to the long-term planning and work They engage in.
If demonic activity is seen as something more like a technological process or making a 'drug' maybe this models gets a bit closer. However I think we also need to guard against making it a one-sided 'materialist' thing as if human souls could really be treated as drugs or a fuel...
Clearly, there must be some kind of 'consent' induced for this to happen - for the energy/ vitality transfer to occur. The victim must be induced to consent.
That, of course, is the Big spiritual problem of these times - the apparent large majority who consent to the evil vampiric operations being performed upon them.