That's a hard question to answer, and one I try to avoid answering because every body has their own (and usually very sensitive) opinions about every way out there that has ever been suggested as the right way/best way to do this difficult deed... some quickly cut the heads off with a sharp pair of scissors or shears, some use blunt force to the head, some use the freezer method, some use starting fluid/ether on a rag and put the keet in a sealed container, some use car exhaust in a sealed container, some take their keets to a vet, etc etc etc. All are horrible things to have to experience and follow thru with, and there's always a huge margin between what's the best way for the keet and what's the quickest and easiest for the individual that is putting the keet down (no fine line IMO). Of course doing it as humanely and as quickly as you can is always the best way for the keet... but not always that easy/doable for the poultry keeper. So whatever works for you personally (and your conscience) is going to be up to you.
Again, sorry you have to go thru this.