well the flash backs are like a testimony for why they are now where they are.... those sinful deeds on which they are being judged.
there are a few reasons for my mental.... granted not all the characters (as far as we are allowed to know) appear to have gotten to the island thru a mortal demise; but others who i shall not name, do not appear to age at all over decades, whilst other characters are portrayed by child actors and then the more familiar adult characters.
it would also explain for why Locke is suddenly able to walk and a certain 'other' never farking dies.
those characters that do die / leave the island however, have judgement made upon them and therefore progress either into heaven or hell (that is for us to determine which is which by way of 'flash forwards')
just a thought!