Just some items to work on
It is not too bad.
Just some items you might want to focus on in future creations
1. Show the eyes of your characters. It is really useful and it allows for your characters to show more expression. It's never really a good idea to cover their faces with hair simply because it detracts from the emotions you may be trying to convey.
You don't need to go the traditional anime style eyes either. Just something so we can see the real emotion on the character's face.
2. Just clean up the art a bit. This is only a mild complaint but the swords seemed more like sticks instead swords at points in the movie.
3. The storyline seems a little off. They are brothers fighting, clearly something happened to one of them. The one brother fails to stop the experimenting on the other then they have the epic battle.
But then it's confusing. Is one fighting because he showed up too late and failed to stop the experiment?
Is the one experimented on know what's happened and simply defending himself, he seems sad which would be at odds if he's some evil minion now?
All in all, a decent flash.