Aggression, fear, dialated eyes convulsions and death. Not pretty and quite dangerous because it can be transmitted to you.
If you suspect that a cat has any severe illness don't waste time, get it to the vet and get a blood test.
There is a 4 day rule. If the cat is so sick it stops eating, 4 days later it will be dead unless there is some serious intervention.
If it has not been outside, no way to get infected by fleas, ticks, rats or a possible bat or squirril then it is unlikely it has rabies. You cannot be 100% sure, get a vet to look at it.