Michael on your comments on the narrow path between bullying and caring and then the analogies to childish behaviours. I totally agree.
I always reminded myself that Dad's behaviour had to be taken with the thought in mind that he had reverted somewhat to the behaviour of a child. Thus the petty getting back at us by tipping his coffee on the carpet!
He also gets nasty if he doesn't get enough attention and has whacked me in the face for not paying attention.
What are young children all about, ego, ego, ego, me, me, me, no wonder dementia caring is tiring!
So yes that path is very narrow, because you have to kind of tread a line where you still make it clear that certain behaviours are not appreciated but you have to also be aware that sometimes or even most of the time, the dementia sufferer cannot help their behavior. Another important thing to remember when treading this line is that although it is good parenting to punish a child for their 'bad', 'selfish', 'stupid' behaviour as this helps them in the learning and growing process, for a dementia sufferer often, all punishment does is make them feel bad, and lower their self-esteem further. And when I say punishment I mean even simple things like yelling at them for being 'stupid'.
For example, if my Dad would not sit down for my mother she would start with asking him to sit down, then raise her voice, then yell, then shove him into a seat. All the while her stress levels would be elevating, Dad would be getting more confused by the second, if I was watching I would be upset, the whole thing was a complete debacle all of the time.
My approach on the other hand would be to ask Dad to sit, if he did not do it immediately I would let it go, if I really needed him to sit, I would while chatting to him about why I would like him to sit, guide him physically but gently if he appeared not be getting the picture. If he still did not want to sit and the situation was not life threatening and I could compromise (i.e. feed him his dinner mouthful by mouthful as he passed by) then that is what I would do. Thats not to say you should let someone with dementia constantly make life harder than it already is for you their carer, but I found most of the time, the desired behaviour wasn't about making life easier, but simply was a habit or convention and there is actually something relaxing about ad libbing and doing unconventional things like feeding a man who walks past you every minute or so!
In ways dealing with Dad is like using your artistic side, approaching things from a new angle and one can take quite a bit of delight from achieving the seemingly impossible via unconventional means.
By the way I have found when Dad does things that are just plain mean, like hit me for not paying attention to him, the tactic that works for me is to turn away for a bit to regain my composure as I suffer both pain from the physical and pain emotionally. Then I simply tell Dad that I wish that he wouldn't do that as it hurts (in a calm and rational voice not accusatory), hold his offending hand lovingly but yet raise it to where he hit me, and then I tell him also that I understand if he can't help it.
[Bear in mind that my Dad can't tell me why he does the things he does and he can only hit me if I am not paying attention and being careful, otherwise he is too slow, uncoordinated and his blow is not very heavy.]
This is just what works for me and Dad, and I offer it only as a suggestion, aware that everyone is different in personality and symptoms.