If you have an oscar already, boom - problem solved.
Any aggressive cichlid would do well with live hunting/food scenarios. I know my convict loves it.
For a large group cull, dump the fish into a very near freezing bowl of water. The system shock should cause organ shutdown and death within seconds. This works very well for smaller fish and it's easy to just dump the entire batch afterward, either down the drain or onto the lawn.
For larger fish, the fastest method is simply crushing the head, but it's not for the weak-willed.
Some folks simply use a running garbage disposal. The fish are dispatched quickly, and it's essentially the same method used for culling male chicks in the poultry industry.