What is the best and quickest way to kill an animal?

I had to kill a baby bunny today because when i was out walking my dog i saw on the road home a car hit and bunny and drove off. The bunny was fliping and writhing in pain unable to move properly. I have just graduated from doing my NDAM at college which is an animal management course and remembering my welfare consideration cles i decided i couldn't just leave this poor rabbit but take it home. Once i got it home i found it was bleeding from the inside and knew it was in a lot of pain and wasn't gonna last very long so i decided to put it down (not it was 9 o'clock so i couldn't take it to the vets). I had my grandad put the rabbit down as i couldn't bare it, But he had two attempts in doing it which redued me to tears. So i was wondering if i get into the same situation again what would be the best way to do this, maybe something like an axe and its neck.