I think I'm going to date my ex's ex-bestfriend.
What is the reason for you wanting to date your ex's ex-best friend?
I'm going out with her to fuck up her relationship for myself.
There is no reason for you to date someone just because they are ex best friends with your ex. All your doing is stirring a pot that does not need to be stirred. If your ex is your ex, then just leave at that and move on.
I know, but I'm attracted to her and I'll do whatever it takes to get her.
Even if you're attracted to her, you must know it's wrong. Just go date another girl that you find attractive.