Depends on the manner in which you choose to dispose of them.
OP, these aren't directed toward you. Just my general thoughts on the subject.
If you sell the fish to responsible keepers, IMO there's nothing wrong with it.
If you trade with someone, that's cool too.
If you use a growout as a feeder fish, I personally have no qualms about this. I'm guilty of it myself. Often unintentionally, but not always. Troublesome centrarchids are the ones who found themselves in this predicament most often. Also, a pearl gourami that seemed to think it was a piranha when it saw fins.
Killing it just because you're tired of it? Not cool man, where's your sense of loyalty? That animal depends on you to live, and you kill it? That's disgusting and a waste. At least feed it to your big fish!
Releasing it into the wild? Especially when it's a species that can thrive in your area? That's really low. Just kill the damn thing.