I despise my best friend's girlfriend
Why don't you like your best friend's girlfriend?
She acts like a slut.
You don't have to be friends with a friends so but you shouldn't despise them, it would seem like you are jealous and that is the reason you don't like them. It will harm your relationship with your friend.
It's okay because we'll all be dead within 10 years anyway.
Don't be so dramatic. Jealousy doesn't look good on anyone.  You only come off as bitter and angry.