"...hasn't anyone consider it may well be very ill?"
I considered the same thing. And/or, he might be in pain. I would imagine a vet either will, or already has checked him out.
There's also the possibility that he's simply a mean cat. Dogs certainly can be. Because no one seems to know anything about Grumpy's former life, there's no telling if they'll be able to help him. Here's hoping!
He may very well have a medical problem that accounts for this behavior, and it won't necessarily be found by the sort of examination he's likely to get from a shelter vet.
Years ago, there was a fairly highly ranked pro tennis player named Helen Kelesi, whose frequent McEnroe-like tantrums were legendary, earning her the nickname "Hurricane Helen". This behavior went on for years, and everybody just assumed she was an arrogant jerk (like McEnroe). But eventually she started getting bad headaches and was discovered to have a large brain tumor, which had likely been the cause of her erratic behavior.