Humans have an uncanny ability to be blinded by cognitive dissonance. When you see a cockroach the first thing you might think is disgust, and then the urge to kill the roach without mercy. Human's can quite effectively apply the same filter to groups of people, think of the 3rd Reich, a whole generation of civilized, well educated, citizens drank the cool-aid of a madman and literally pledged to wipe certain ethnic groups off the face of the earth. When these officers brutally beat protesters it's no different then when you wack a roach with a shoe, your disgusted by it's presence, you see it as an invasion to your personal space, and without remorse you do everything in your power to destroy it. This is what you are witnessing.