Can I be friends with my ex?
How do you feel about being friends with your ex?
She does not deserve my friendship for what she did to me
That's good that you can both recognize the behavior of your ex was problematical. Even better that you can rise above it. Like they say, it takes two to Tango.
Why do they say that ?
It means you both are responsible for the current situation not just one of them. It's cool if you want to be friends with your ex  it's cool too if you don't want to.