Wow. I never thought I'd see a topic like this.
It would take millions of years to change their behavior.
It took millions of years to make them what they are today.
Being aggressive is their nature. And changing that would be changing something that happens in nature in all sorts of species.
Lions, Peacocks, Bettas, Turkeys, the list goes on and on. Don't you know why male bettas are so pretty? They have to impress the ladies. Just like so many other species.
And like all those other animals, the biggest, prettiest, and strongest betta always gets the girls.
It's what makes bettas so interesting compared to other fish. I like seeing my bettas get all sassy and strut their 'stuff'. Their whole body language changes when they meet another betta.
That's what makes them fun!