How then could a department get rid of a bad officer before this bad behavior presents itself?

The public only becomes aware of the bad behavior when it goes too far and, for example, a cop unnecessarily kills somebody.

But that doesn’t just happen out of the blue. That cop’s coworkers and supervisors knew that he was reckless or emotionally unstable or whatever loooong before it got to that point. They need to step up before he kills somebody.