People who are usually considered to be in positions of authority typically have training in how to not react. If I had to have it as a person who saw patients (although all I did was talk to them and listen), cops would have had it. We had classes, and we had exercises involving pretend patients who were stressed, unreasonable, and intentionally provocative so we could practice not being stupid.   Perhaps it's not surprising that some people still got upset. The police, I'm sure, have been trained how to NOT escalate situations or allow others to do so.

This cop lost it. I watched the video, and thought that, at any time during the whole confrontation, he had treated her as a fellow human and just gave her the impression he had a little bit of respect and compassion, the situation would have been defused. Seriously, when someone pushes you, and YOU have the gun, the handcuffs, and the power of the law, you quit fucking pushing back.

Yes, she could have stifled herself, but police have to not allow people to get under their skin lose control over themselves, and simply react. That's what the Internet's for.