Originally Posted by Hershey
I can't remember the video name..
Ok, well the explanation for this is that bettas are not aggressive due to a naturally evolved aggressive streak. They were artificially selected for the trait over thousands of years by humans who wanted to fight them. Any betta species that cannot interbreed with betta splendens is generally pretty peaceful given the right amount of room and plant cover, but species that can interbreed with betta splendens may have some of those aggressive genes in their gene pools due to the aggressive domesticated bettas being released back into the wild by owners who didn't realize what they were doing to the native wild populations.
It is probably impossible to find a truly wild betta splendens with absolutely no genes from a domesticated betta, but if you were somehow able to find one, it could probably live with other bettas and fish without being at all aggressive.