Killing a Animal to prevent painful of that Animal.Is it consider as a bad thing for me according to buddhism?

Note : Sorry for my bad English. If anyone can please edit the question for better meaning. There is a dog. But dog is spending very painful life after huge burning(just before 10-20 minutes ago). It is very painful. I repeat it it is very very painful. So if I kill Dog, is it consider as a bad thing for me according to buddhism? If I do it, I will do it just only stop painful of dog. So is it consider as a bad thing for me according to buddhism?