This is uncharacteristically charitable of me, but rather than “mentally ill”, I consider such people profoundly, tragically human.
One Facebook user commented “I honestly never understood the joy some of these people get from the thought of people being tortured forever”, and I think this exemplifies the question, of which the operative words are ‘tortured’ and ‘forever’. To me, simply, statements like those in the OP demonstrate the massively overwhelming inability of humanity ever to come to terms with its own transience. That is why an eternal and blissful scenario must be constructed, as a palliative, of which the obvious flip side – roasting forever in a lake of fire, or something displaying similarly gruesome creativity – serves to reinforce (read: assuage/reassure) the psychological defences and cognitive limitations of those who subscribe to that particular construct (or who have been inculcated into it).
If this outlook is indeed a mental illness, then it is one from which the majority of people in the majority of civilisations in recorded history have suffered, so I would tend to see it as “low-hanging fruit”. As long as such ideas harm no one other than those who entertain them, I’m relatively resigned (“happy” would be overstated) to leaving people to their delusions.
Normal cynical service resumed shortly.