I'm going to tell my friend that my other friend was talking about her behind her back
Why would you gossip about your friends?
I want to be a bitch and a backstabber.
People use gossip for their own selfish interests at the expense of others. Mind your own business and let the friend sort themselves out without your input.
Whatever, I'm still going to tell her.
What are you hoping to gain by doing this? Being a bitch might be fun in the short term, but in the long term you'll lose people's trust and you'll feel lonely with no friends to confide in.