We, like you, don't like to have to destroy a fish. However, if we kept every fish that resulted from a spawn, we would not be able to keep up.
Why don't we keep all the fish? Economically, there would be no way to sell them and even pay for the food it took to raise them...that would be one reason. The market for good fish is fine, but the market for average fish is not economically viable in our configuration.
Another reason might be that some of the fish will be "runts" as a result of normal genetics. All fish hatched in the wild can not be expected to survive for the same sorts of reasons that we might artificially decide which will live and which will not. Because we have raised the fish outside of a natural environment, we now need to face the necessity of making some "un-natural" decisions.
So somewhere along the way, sometime during the process we have to make decisions. Sometimes the decisions are easy as in the case of disfigurements and stunted growths. If we have followed our normal course of action and thought about our goal before the spawn, some of the decisions become easier. Sometimes a color is undesirable. There are times when finnage type is the goal. Whatever the criteria for culling is for a particular spawn we make sure that we keep focused on the goal. If one does not keep focused on the goal, then the possibilities of creating average fish is perpetuated and in a bazaar sort of way, perpetuating the betta-in-small-jar-that-never-gets-a-water-change situation.
More or less, going back to the "have a goal" thought, if one does not have a reason to induce the fish to spawn...don't. If one doesn't have a way to move the fish into new homes...don't start the process. What we don't want to encourage is for folks to become the "puppy mill" of the Betta world. That wouldn't help the hobby and certainly not a good thing for the fish or the species and hobbies future.
You might be wondering how we dispose of the fish that do not get chosen to continue in the program. It takes a lot of sole searching to come up with a solution or a combination of solutions for these fish.
We have a number of larger fish that we feed fish too. Some of those fish-for-food are feeders we purchase and some are well...results of the culling process.
In the event that we have to use other methods of disposal we use a second method. We have read a lot and thought about it a lot and decided many years ago that the best method was to freeze the fish.
When the water begins to cool the metabolism of the fish begins to slow. Eventually as the fish gets cold enough, the body shuts systems down and the fish "goes to sleep."
This notion of freezing has been a lot less traumatic than any other method we have read about.
While we do dispose of fish as needed, we don't like too. In order not find oneself in the situation where decisions have to be made, it's best to start with that original premise (the Goal) and not to just see if you can make it happen (that's the easy part).
Good notes help.