“Above all” vs “after all”

“Above all” means “the most important of all”. It aims at telling readers what follows is the most important point or idea.

e.g. You should stop smoking. It simply wastes money. Moreover, you will lose friends. Above all, it will kill you.

“After all” implies concession. It means something like “Don’t forget this”.

e.g. We should forgive John for his rude speech to David. After all, he was very angry at that time.