I am thinking that it may be a good idea for me to get rid of a dog that my girlfriend is trying to adopt.
What is your reason for thinking about getting rid of this dog?
I hate dogs and I know my girlfriend wants a puppy so bad.
I'm not quite sure what you mean by "getting rid of," but it's wrong to do anything cruel to an animal. If your girlfriend wants to adopt a dog, she will be mad if you mess that up.
I'm talking about killing the dog.
Well you should definitely not kill the dog. It is not the dog's fault that he/she was adopted. What you should do is just be honest with your girlfriend and let her know that you would rather not have a dog.