umm ive only felt bad about killing a fish inhumanely when some kid who wasnt with me walked around the corner just as i hit the fish with a rock, kid stopped dead in his tracks but oh well. either leave them out of water for a few minutes, hit them with your knife, or a rock. its how ive been tought to do it. but ALWAYS the same day i catch them lol, though if i get them at night or with too little light ill leave them in the lake overnight