I probably did sound a bit callous, but I guess we have our limits. Looking after someone with dementia is dispiriting enough, and increasing hard work as the dementia worsens, so I guess having to be 'the court jester' and cheer them up constantly is just on the 'too damn much!' side when they seem to 'insist' on being miserable ,and expecting YOU to do something about it!
You raise the disturbing question of whether that 'spitefulness' is inherent but masked by social expectations, sort of along the lines of id/ego/superego....the idea that we would ALL be 'pure id' if we actually could get away with it!
That the 'spitefulness' is a form of the dreaded 'disinhibition' that dementia sufferers can show (I can remember David Baddiel the comic saying how embarrassing it was that his father with dementa became highly disinhibited around young female nurses...gross gross grosss!), but applied to 'personality' rather than physical displays....
It would be nicer to think that maybe dementia only unbalances our for-want-of-a-better-term 'Manichean' or 'Ying/Yang' mentality - ie, if we assume that we are all 'equally likely to be good or bad' and could go 'either way' in life (usually depending on how we are brought up of course!) (eg, could Hitler have been raised to be a nice guy etc??) - and that all dementia does is 'tilt' the usually balanced mind into the 'bad' side of the moral weighing scales!
Maybe another way yet is to think of dementia as bringing out (sometimes!) our 'inner psychopath'! Maybe that's akin to the 'pure id' personality (and in a way, I guess that who psychopaths are - NO concept of 'other people' at all!) (except as mugs and victims and slaves)
In the end, I suppose it doesn't really matter what the cause of the altered 'spiteful personality' in dementia is - it's how we deal with it.
I would say cut SOME slack (in the same way that someone in pain can be 'tetchy' so we cut them some slack!), but not excessively so at a high cost to yourself.
To my mind, if the bottom line is they can't be happy anyway, and nothing you do can make them happy, then why bother putting yourself out at all for a 'lost cause'????