I really think your fiance should get his priorities in order. You should be #1 in his life now, and his soon-to-be-born child. Does he think it would be OK if the dog attacked you and hurt you? just because the dog is old, in pain and thinks she should do what she wants? The staring doesn't sound good at all especially with Bella's increasing aggressiveness. Apart from living in filth as you describe is not something you should have to be subjected to, especially if you have to clean it up. It is obvious that this poor dog is suffering with pain with this large tumour and it's making her feel cranky. Your fiance is being cruel in allowing her to go on in this state. I agree with the others that it is time (past the time!) to have her euthanized and let her go before something worse happens.
"We humans are indeed fortunate if we happen to be chosen to be owned by a cat." -- Anonymous