How much damage could your best friend do to you if you ever had a falling out?

People can be very vindictive when they call themselves getting revenge on you and if there are friends out there like that who needs enemies. Someone who would intentionally go out there way to do any sort of damage with the intent of harming you is not a friend.

Talk really bad about me to another enemy

It could break me, I would literally be to myself for some time.

My best friend is my fiancé soo definitely a lot of damage

I’d be less open to trusting and making new friends.