Skyline

So I watched the movie Skyline after my Dance Company performance last night and there was a scene where one of the main characters died not because they were weak, it was just by chance their body couldn't handle the pain so they ended up dying. 

According to Darwin only the strong will survive because they are the fittest. Now, my question is what happens when someone dies unexpectedly? Is it because they weren't fit enough so they were rooted out or was it just chance they happen to be crossing the street when a man drunk off of road rage forget to stop at a  red light and kills them. They didn't die exactly because there weren't the fittest but it was just by chance they did. Also even if the strong survive we all have to die at some point because we can't live forever so is it apart of us we are all destined to die one day.