I'm finding out that I'm incredibly attracted to my best friends boyfriend
What is attractive about your best friend's boyfriend?
I just want to steal him from her by acting nice to him
The best way to keep it safe and healthy when you have romantic feelings with your best friend's boyfriend is to tell your best friend your feelings. Communication is key to keeping your friendship and your sanity healthy.
I know what to do. I'll just act nice towards him and he'll be mine.
Don't you value your friendship a lot? I feel like it would be better for you to try and take a breather away from them for a while if this pains you too much. Your best friend deserves both a wonderful friendship with you, and a happy romantic relationship.