I can see why the "bully" is really the victim who will run when confronted. OTOH, it's been the pussy victims of bullying who have shot up schools and other places. They can, indeed, be violent even though most are not...at least not to adults.
Without a psych eval, it's difficult to tell the difference between who will and who won't...and even that isn't perfect.
In the 70s there was a page in MAD magazine which showed drawings of five young women. It asked something like "Which of those women are sweet and which are psychos who'll knife you in the back?" Then, at the bottom of the page it said "When you find, please let us know."