I'm going to tell my ex she should stop seeing a guy with sociopathic tendencies
Why does it matter to you who she decides to see?
He's a really scary guy and he scares me.
I understand you still care for your ex but her life is not your business anymore. She's an adult so she's competent enough to make her own life decisions. You should move on with your life.
I can't stop caring about him, I totally want to freak him out by twisting things he did in my mind and making them look more serious than they are.
You should really just let your ex make their own decisions in their life. It is fine to still care about them, but do not worry yourself about things you can not control.