I feel like "can't do it" shouldn't be an excuse for anyone to extend the time it takes to kill their fish. If it becomes necessary to kill your fish (or anything, for that matter) you shouldn't be concerned about how it makes YOU feel, instead you should be thinking about the quickest and most painless way to dispatch the poor creature. Anything else is incredibly selfish.
Put yourself in the fish's shoes. If you had a choice, would you rather go out quickly and painlessly or die a slow death in the freezer because someone "wasn't comfortable with killing?"