A good big rock slammed down on the head.
I know it feels sad to do that, but if you havent wrung an animals neck (and I havent) it may be difficult, especially on an animal thats injured, they can react violently.
a friend of mine recently ran over a rabbit. It was in that much pain it was literally screaming. He rang everybody he could to get someone to come and kill it because he was scared to do it himself, but it was suffering so much he couldnt leave it. He was crying when his mum got there and the rabbit had died from its injuries. He felt really guilty afterwards that he wasnt able to do it himself and end its pain, so its good youre asking these questions, I suppose its useful to know the best answer.
sorry m8 but that is verry inhuman and i dispach quarry for a living like i say the best yaw is ither a nife strat to the hart or kneck them