I think my best friends girlfriend is into me
Why do you think she is into you?
Yeah, she's like a slut. She's very lonely and desperate.
You should not call her that. If she is into you, then you need to tell your friend about this. He would not like being cheated on, especially with his best friend.
I wasn't thinking that far ahead. I'll keep it to myself and when they break up, I'll swoop in.
Keeping it to yourself is not good behavior. If that is your best friend you need to let him not and not swoop in when they break up. That is a large breach of trust that can end the friendship.