In many, if not most, of the best short stories, the conclusion is inexorable. The tale advances elegantly, carefully, constantly toward its destination—no detours or loose threads. The theme unfolds, the climax arrives, and the final sentence reverberates because it rings true to every word that came before it. The story can end no other way.
In its first two-thirds, I Am Legend feels like one of those short stories—beautiful and relentless—and if the movie only ended at the darkly resonant sequence that caps those two-thirds, it would be a brilliant, brutal cinematic short story. But it doesn’t end there, of course. It spins off into something safer and less interesting. It’s not bad, exactly, but the jarring shift in tone and theme (not to mention quality) make the ending a disappointment. The rest of the movie is compelling enough to make it worthwhile, but the thought of what might have been is hard to shake.