An old roommate and I got a similar Idea years ago. We were both young and naive. We were at wal-mart, saw the betta display and thought it would be funny to buy two males and bring them over to our buddy's house who was hosting a barbecue.
Young males, inebriated with alcohol, never having fish before, myself, we decided to put the two fish into the same bowl with eachother so we could see what woudl happen.
We thought the flaring was amazing. We laughed and cheered at the "fighting" and the tactics. We didn't place bets, but we were entertained. Because they were fish, and at the time, looked at them so simply, we didn't care about any morality involved.
One of my buddies there is a major humanitarian, and a lover of all things living. He did not tolerate this at all, and we eventually separated the bettas, and put them back to individual bowls. I can't recall what happened to the bettas, but I know they didn't live much longer, whether they were flushed down a toilet or disposed of in a less then ethical way, i don't recall.
I never recollected on that experience until after I got my bettas that I have now, whom I love and care for deeply like any other pet that I've had before, and read this post, and wrote my own response.
It isn't right to play god when you aren't, and it definitely isn't right to put two unknowing individuals into a match where there are no rules of safety in consideration.
I hope you learned from this in a wholesome way. Just because they're smaller than you in size, doesn't mean they aren't bigger than you in weights of karma.