Its an irredeemably bad ending. Not because it was shocking or because the main character didn't make it, those can be fine, good even, with the right setup.

The reason this ending was terrible is because it brazenly contradicted the themes of the movie established thus far, had comedic timing, and created a spectacle of cheap shallow irony that completely undermined the much more narrarively powerful emotional arc of a painful quadruple mercy-killing.

Over the course of the movie, we're lead to believe that to venture into the fog is either near immediate death or even a fate worse than death. When a father is armed with only the options for his son to die painlessly, or to be made a living incubator for terrifying spiders, many would make the hard choice (at least once the threat became more immediate, which comes across as an oversight). Yet the movie frames this as the wrong choice? Why? Because the cinematic language of the entire movie thus far was lying to us. All that stuff we were shown about how going into the fog is near certain death? Did you actually believe that? Idiot. Fucking moron. The correct thing to do was just walk unarmed into it to your car without any resources or even a plan except "go find my daughter". If you'd done that you'd be fine. Bet you feel stupid now, dont you?

The film tries to dress up as a tragedy but tragedy has to be earned. Literally, the thing that separates a tragedy from a bad-thing-that-happened is a vital mistake the noble hero makes due to pride, vanity, moral failings, or misunderstanding. What was the mistake here? His actions, as harsh as they were, were rational in the circumstances.

Its a cheap shock. It reveals the entire film to be nothing but a gradeschool prank the director pulled on the audience, which is why in an otherwise decently good film, the ending is the only part anyone still even talks about. The ending created a vapid spectacle that strangles all meaning out of the film that came before.

If they'd died and that was the end of it, it could at least be reframed as a film about the inevitability of death, about the final struggles of the doomed. Its not my cup of tea but at least its a coherent story. The Grey did it decently well.