hey everyone,
it's me again.sorry to bother everyone again.but i was hoping anyone knew any animes beside gundam seed and x that two best friends fight in different sides and become enemies.i'm doing research for a superhero story i'm working on.
thank you all for your time.
-Joe:)

I can only think of small bits in series when old friends fight each other:
Theres a small bit in full metal panic where souske meet up with a friend from his childhood as a millitary gorilla and has to defeat him, this last three episodes.

Another case is in Rahxephon, the main character fights a giant robot thinking that he is protecting a girl from his child hood but is in fact attacking her and in the end kills her, another point is near the end where he has to fight his friend and cuts him in half.

In orphen orphen has to fight a person who is possesed by the soul of his adopted sister who was transformered into a dragon and fought orphen.

These are all I can think of if I think of any more I'll post.