I just felt the ending went against everything that made the rest of the film work. Yeah, there's the cliche comment about how "the real horror" is the malleability of our established ethics, principles and sense of rationality in moments of crisis, but also there's like, the crisis itself of a Lovecraftian nightmare dimension bleeding into ours and affecting the world in a manner completely beyond our comprehension with the thick mist entirely obscuring our field of vision. The main overarching horror of the film - especially once the characters leave the grocery store - is that the horrors we've seen are a mere microcosm of the situation, with new discoveries like the pharmacy creatures and the behemoth at the end exacerbating our anxieties about the gravity of the situation, and ultimately Frank Darabont's "twist" sacrificed that sense of uncertainty and wonder all for some (insanely far-fetched) tragic irony.
I disagree that it works against that. I'd argue that that very premise of them only seeing the very beginning of the horrors, that their anixeties of the unknown have become so large and overwhleming, it makes that decision all the more rational. Not to mention we have no guarantee that the world is just completely better after that moment; the military has just cleaned up that initial area. They very well could have only been safe for a little bit. But it's that toying with what happens when you've lost all hope, act on that loss of hope, just to have that glimmer of hope return after it's too late that makes the ending so powerful IMO.
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"She was like, 'Oh, did you see that firefighter? Hes so cute.' And I was like, Mom, I just got blown up."