I mostly ignored Dead Space when it came out, but last year I played the remake and then just recently finished up 2 and 3 and 3's DLC, which it turns out (I did not know this) is essential and is the end of the story, and it's a really good ending. This was a while ago now and it's really stuck with me. I think it must be the grimmest game ending I've seen, super horrible, super good.
It's so rare in games not to get a happy ending, and happy endings I feel are usually sort of weak. My favourites tend to be more bittersweet, and it's slim pickings for anything actually bad, like you failed, like the world died, which is what DS3 is. There's something kind of weird in having the hero repeatedly overcome incredible odds because they're a video game protagonist, but then suddenly not - like your suspension of disbelief is being taken advantage of, I'm not sure whether on purpose or not.
This reminds me of how Marc Laidlaw wanted to end the Combine arc of Half-Life with the reveal that the Combine is so overwhelmingly vast it's literally impossible to put a dent in, and all you can do is make yourself hard for them to reach, going into an HL3 that has nothing to do with them, but they still exist essentially as a distant angry god. That's basically the setup for Dead Space 3, and then the worst possible thing happens, and it's perfect - you went on 3 horrific adventures with the worst never quite happening, and then the worst does happen in a very uncompromised way. Whole world's fucked forever, thanks for playing. I like this game more the more it lives in my head.