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.

Yeah, the book vampires were much more fleshed out. In the movie they were just barely-sentient beasts, primarily running off of instinct. They only seemingly had some basic higher-level reasoning. His primary struggle was surviving while surrounded by bloodthirsty animals.

In the book, they were a full blown society with their own culture. When the people around him changed, he was suddenly a stranger in a brand new culture. The point was that in the old society, vampires were the thing that went bump in the night. But in the new society, he was the monster that parents told their kids to watch out for.

It gave the illusion of following the book.

Have you actually read the short story? Because I am baffled as to how anyone who has read the story would say that.

The movie was in no way an adaptation of the short story at all. It never even pretended to follow the short story.

Just like iRobot the only thing I Am Legend has in common with it’s written work is the title.

He is then executed.

No he wasn’t. He committed suicide.