The real problem with the ending, as someone said upthread, is that it ends too abruptly and with the army showing up RIGHT THEN. Here's the gag folks - the goddamn LINE OF TANKS was trundling along /right behind their truck/? That's not horror, that's comedy and cruel, sadistic irony.
And that's the problem. It cheapens the shock of what just happened to have a cartoonish "gotcha!" 30 seconds after the deed is done.
When he got out of the car after shooting everyone, for a moment, I had a gut instinct on what was about to happen. This is the scenario that popped into my head instantly:
He's screaming at the monsters to "come on!" and finish it. But they're not coming. I was /certain/ that the real twist of the film would be that he would not be killed and wander in the mist, wondering why he's not being taken. I expected the film would go on another 10 minutes or so, until finally, he comes to some kind of clearing in the mist and walks out, either finding the army or just by himself, and is forced to contemplate the real horror of the situation: how none of it was personal. The monsters weren't evil, they were just animals from an alien dimension. He wasn't killed due to blind chance and the worst part - he couldn't say he did the wrong thing by shooting the others. That's just the point - the universe is random. He is left alone as the survivor not because there's a lesson to be learned or a message for him but... just because.
So when the tank rocks up as soon as he steps outside, my jaw dropped. Literally. I said "you have to be shitting me" under my breath. So cheap, so stupid, so caving in at the last moment to create a cheap, cruel twist ending.
It's proof that great films can be soured by the final scene.