people say a blender would be inhumane, and I've never tried it myself...
...but if you don't mind mess, I reckon it would be an almost-perfect method of killing. It decapitates and destroys the brain and body in less than 2 seconds, thus little if any pain. Perfect! If I had an old blender not used for food, I'd consider it if I had a sick fish beyond help. If you added some water before switching on, you could simply empty your liquified fish down the sink afterwards, rather than scraping it off the inside as you would if you just put the fish in.