I would have been concerned about the cats too, Bobert. I don't know anything about snapping turtles, but from what you describe it might have badly injured a pet. It died instantly and did not suffer. Sometimes in life I've regretted and beat myself up about things I've done, but eventually you have to accept that what's done is done, and let it go. I like very much your concern about the rightness/wrongness of your action. You obviously have a caring personality.