Attempt to sacrifice likeable character in attempt to salvage loathed one is bound to backfire..badly.
Except that that is not what is going to happen. They do not kill Sarah in order to make Laurel Black Canary (if they do it at all), but IF Sarah were to die (and frankly, too many characters having plot armor is going to get old with one more CW show, just like Vampire Diaries/Supernatural), that would completely turn around Laurel AND make perfect sense to make her start on a new path. The "cause-reaction" chain is messed up with us peaking behind the scenes, that is not even hinted in the show.