This thread has brought back some really nasty memories! Mine are now teenagers, so we don't have this problem anymore, but my bf when mine were little had the child from hell! He wasn't autistic & there was no reason for his appalling behaviour except that he was spoilt.
My house is horribly messy- I'm not in the least houseproud, so ordinary playing-with-toys mess never bothered me at all. He would arrive, empty the toy box upside down & spread everything across the floor. (Ours just took out what they were playing with- a dozen or so on the floor is fine, but whole toybox?).
He'd then start demanding to go upstairs & I'd tell him no. (All toys in livingroom). He'd keep on & on & eventually sneak up there on the pretext of needing the loo. I'd find him up to no good, poking around in one of the bedrooms- usually mine.
One time I put the stairgate up before they arrived. His mum said "he can climb that" !!!!!!
He would break stuff on purpose; take food out of the fridge/cupboards without asking (having been given drinks & biscuits); you get the picture. He would also demand to see the baby & be practically climbing in the pram & pulling at his legs. Beccarol- the boy you describe is exactly how this dear little soul used to be with my younger boys.
His worst trick was to slyly poke, slap or otherwise antagonise one of my kids, then when they retaliated run to mummy. More often than not his mum saw the retaliation but not his initial attack. We almost fell out the day she told off my DD for defending herself!
She used to tell him off, and shout at him quite a lot at their house, but never seemed to be aware of his worst behaviour. I was relieved when he started school & we no longer had to put up with him. His younger brothers were fine.
(All this was 14 years ago but it's making me angry now to think about it!) I still have problems telling off children when their parents are there. I suppose it is a lack of confidence on my part, but also that I keep an eye on mine and make sure they behave, so I expect other people to.