Charlie's devotion to Claire and Aaron gives him purpose beyond his addiction and failed music career. He reads Aaron's horoscope, builds Claire a crib, writes "Greatest Hits" for Aaron to remember him by. His death is explicitly for them -- Desmond's vision shows Claire and Aaron getting on a helicopter, and Charlie dies to make it happen. The tragedy: Claire never gets on that helicopter. Charlie dies for a future that doesn't come true, but his sacrifice still saves everyone else.