In case you haven't seen the alternate ending for I am Legend, it puts a very different perspective on the whole movie. Apparently it was the original, but didn't screen well with viewers.

The most telling moment for me is the infected slaps their hand on the glass and draws a butterfly as the last words the protagonist's daughter ever said to him, "Daddy, look a the butterfly!" echo is his head and he realizes that the infected he has captured has a butterfly tattoo on her shoulder. He even makes a note of it in the capture and experimentation scene claiming that the infected exposing himself to sunlight is a sign that "social de-evolution is complete." when instead the infected just witnessed a monster kidnap his daughter and drag her into a dangerous area that he cannot follow to do unknown experiments on her to change her into something else.

Instead the ending negates everything built up to the point and ends with a boring action-movie cliche.