A pretty appalling story.
I suppose as others have said, if these people had suffered some traumatic event in their past, it could be a mitigation, but still is pretty unjustifiable behaviour.
I guess your response to finding a left rucksack will depend on your experience, and how that informs your analysis of the situation. Experienced walkers, especially 'baggers' would surely recognised that a sack left at a col or the foot of an obvious outlier, has been left there deliberately. Now if the sack was found in the middle of a narrow ridge, that would be of more concern, and I think personally in that situation, I might give MRT a ring.
But to pick up and move a rucksack, in any situation, is surely moronic!
For what it's worth, a few years ago, on an event filled trip on Skye, I lost my rucksack down a crag in Skye (link here for the whole sorry tale
viewtopic.php?f=9&t=54193 ). In that situation, I alerted MRT so that if anyone did ring in concerned, they'd have the details on record.