Why do people slander and hate police officers?

I've seen countless amounts of comments, videos, and word of mouth, the hatred of law enforcement. Why do people hate them so much? They're out there to help us, protect us, and serve us. Yet, people on youtube like "CopsOutOfControl" and **** like that are titling and uploading slanderous videos which gives an extremely negative image of cops. When people need help, they go to the police. When people need rescuing, they go to the police. When there's somebody snatching up your kids, people go to the police. So why is everyone hating?