I don't envy police who are sent to contain or control protests, especially since there are usually protesters (some of them even not agents provocateur) who are looking for trouble in even the most peaceful settings. And sometimes cops are ordered to do things that they might not otherwise be inclined to do.
But there are clearly cops who enjoy the power, who enjoy striking back, who have anger management issues, who hate damned hippies, who don't suffer taunts or protests well, and/or who, in this Internet/mobile phone age, keep getting caught on camera …