My parents' 3rd cat. God, she was a fucking nightmare - and in her 4 short years of life conducted a reign of terror on all of us. Except my parents were besotted by her and couldn't understand why we stopped visiting so much when she was around. Why? Well, she would cosy on up to my son who had grown up with 5 cats, knew to respect them, and was scared of this cat regardless. and bite him just because. She would wait on the landing as my daughter went up to the bathroom, on eye-level with her, and reach a paw through towards her face and hiss whilst flexing her claws at her eyes. And her standard greeting for me was to aggressively hiss. She terrorised the older cat (who thankfully had 8 peaceful years without her) and their dog and mine. We think she ate a false widow spider, the symptoms prior sound like she might have, and she did die in the emergency vets, late at night, by herself (and for that, I do feel pity because no animal deserves to die alone), but all my children and I could say - privately, my parents have no clue we actually feel like this - was "thank fuck!". My daughter is nigh on certain that the cat was demonically possessed...
Ironically, I was with my mother when she chose the cat as a kitten and I was trying to steer her away because she bit my mother's finger hard enough to draw blood.
And, in 45 years of consistently sharing my life/home with dogs, cats, rabbits, budgies, horses, hamsters, gerbils, and fish - this is the only one that I've gone "nope!" about.
With regards to the whining about food... have you had them checked for feline dementia? My old cat, who was 22 when he passed, was diagnosed with this when he was 6 - and he'd literally forget that he'd been fed, even as he turned away from his bowl. He could be an absolute nightmare about it, especially as the years went on and the "holes" in his brain got larger. It wasn't his fault, though. And I coped with it because I'd had him from 3 weeks old (rescue/foster situation). We also had a FD cat who reactively peed everywhere, who was 13 when he passed, and one who was the perfect cat (part-Bengal, also 13 when he passed, went through life believing everyone adored him). It would have been so easy for me to hate them... but so few people realise that elderly cats (and dogs) are just as prone to things like dementia as we are in their dotage. It takes a good vet to diagnose properly, but it's very prevalent in the feline world and more recognisable than it is in dogs from what I've read.