well observe him.. if its mild cuts, it might just heal.
but if you do want to kill him, heres how you should do it.
get a bowl/bucket/cup of water, and put tons of ice in it. make sure the water is as cold as you can get it.. net the fish, and put the fish into the container, and it should be instantly shocked within 3 seconds, and itll pass away. thats the most humane way. DONT flush it down the toilet or anything like that.