WildBill wrote:IANAL nor Ann Landers nor Miss Manners so don't get angry or defensive. You asked for advice. She is not over him. She likes conflict and drama in her life. Get a new girlfriend. .

Aaaaaaayup! If she had your best interest at heart, she would agree that a report needs to be filed. That she doesn't indicates that, right now, she cares more about his future than yours. You only have to ask yourself how she would feel if he actually did hurt you. However, that doesn't mean that I think she's a bad GF. It means that I think she needs to get some help for herself and that she's not thinking straight. Here's why...
You said:
The guy's 18, and has a history of being not physically abusive, but yelling / throwing fits / hitting walls when he was with my gf.
That
IS abuse. It's called
psychological abuse, and it is a recognized form of abuse in a relationship. Psychologists and the courts recognize it as abuse. Worse yet, a person who is like that is only a hair trigger away from actually
physically abusing her (or you). That she doesn't want the authorities involved is exactly the same kind of dysfunction as the physically abused woman who doesn't want her abuser "in trouble" for giving her a black eye and breaking her nose. They think that if they can just love him enough, he'll be nice to her. They view themselves as partly at fault for being abused. I'm sure that your GF is a sweet gal and that you care about her alot, but she needs help. Really.
Please don't take this as a slur on her character or yours, because it isn't meant that way. I've just spent enough years working in an ER, and seen enough abused women to know a little bit of what I'm talking about. She needs a dose of perspective, and for his part, the ex-boyfriend needs to learn that there are consequences for issuing threats of bodily injury. Allowing him to continue to be a loose cannon serves nobody's best interests, including his own.