User blog comment:Jonny16/What is the meaning for Oda having Sabo killed??/@comment-3075024-20111227185937

Similar blog made http://onepiece.wikia.com/wiki/User_blog:Monkey.D.Me/WAS_HE_NEEDED%3F%3F

You will find your answer in that blog but lets me post the best answer from out very own Cut-Throw-Iron-Hart Admin DancePowder -

''He is Luffy's death from the past, just like Kuina and Tom, and those other guys. It could have been Ace, but Oda wanted him to live. It's not uncommon for him to introduce characters just for them to die in a flashback.''

hope that helped