This is a major problem in the system. We all agree we need officers to protect society from unstable, dangerous, violent people. But we also need to have a method in place to ensure that the officers protecting us don't have anger issues, mental issues (manic depressive, obsessive compulsive, bi-polar, split personality, socialpath, etc). Too often when I watch these videos it appears that the officers allow their own feelings to get involved, making it a 'personal' thing for the officer and they react out of anger, vindictiveness, or sometimes just over grown egos and they enjoy being bullies.
This is on the police level and not higher either. If I stood on the street with a sign that said 'This government is run by a-holes', nothing would happen to me. If I yelled out to the Prime Minister 'Hey, you're an a-hole', nothing would happen. If you say it to a cop, some will make it personal and want to beat you.
They need to be made more accountable for their actions, and the police departments need to weed out the bad ones and in the future cops could be viewed as heroes like firemen are.