When you mentioned cat soup, I started thinking about something that had to do with cat soup and ketchup (catsup) but I'd rather not go there!
Yes a dark twist might be the only way... unless it's hilarious. If the body is funny enough, the audience will forgive any ending. The critics will not (forgive it) but I guess you can win Cannes or you can win MTV but you can't win both. (Doesn't mean we can't try.) Of course, making it funny would present a whole new challenge that depends a lot on luck or finding someone who is really naturally funny to come up with stuff.
In the twist avenue, the (very) old lady could be walking by the door with a bottle of milk in her hand when she hears a meow. She looks through the peeping hole of her heavily bolted door, since her windows are boarded.
Outside, kitty is looking especially cute and especially famished.
She looks at the milk, she looks at kitty, she looks at the hundreds of locks on the door. Finally she starts opening the locks, opens a creak in the door and puts out the bottle of milk. She tries to pull her arm back in but... it's too late! In a flash, kitty has put a handcuff on her wrist and POOF, transforms back into a grim reaper. He laughs. Finally, he has her! He knew she would not be able to resist that one! He pulls on the arm and it's a fake arm with a stick of dynamite sticking out on the other end. The door quickly closes.
Or another....
In this one all the shots of the inside of the house look cozy and warm and the outside looks especially cold and harsh.
There is only one cat picture but it has no candle since the cat did not die. He had to leave because... I'm getting ahead of myself... well, anyway, at the end when they get kitty inside, the old lady looks more alarmed than happy.
Kitty jumps into her arms and the landlord's shadow shoots into the room. A long, imposing shadow, even though the landlord is quite short. He points at the sign that says, "No pets under penalty of eviction!" and kicks them both out into the street.
or another... though I think I've seen this one done many times before, in different scenarios...
the little animals try to get kitty in the house by opening an entrance and the mice keep preventing them from using it. When finally they can get kitty inside for the happy reunion, the house has been so heavily damaged that it falls down around them.
Another one, on the very happy ending side...
The mice really go to extremes to stop kitty from getting inside. They are terrified of the prospect of having a cat in there and know their lives would be forfeit. After the mice have done all their nefarious deeds to try to stop the animals from getting kitty into the house, it's finally in, joined in a loving embrace with the old lady. The mice are going, defeated and nervous, back to their mousehole when kitty slams his paw in front of the entrance preventing them from going in. The mice are terrified but kitty slides his paw into the mousehole and snaps the mousetrap that was waiting for them inside. Kitty smiles at them. Cut to the three mice sitting together in line, with a tiny napkin tied around their neck and kitty feeding them soup with a tiny spoon and the old lady feeding kitty soup.