Me, my sister and a friend from college came up with these 3 possible answers....plus the 'own idea' thing for you.
VOTE, I SAY!
Post how you think she died. And this isn't serious, just harmless fun!
I chose Cloud. Reason? It should say so in my 'personal quote'.
For those that can't read it:
Cloud Killed Aerith! Sephiroth just knocked her out and then Cloud drowned her!!