They don't really serve us. That's what has them lining up to crush a protest or to break a strike. Or to over patrol a poor neighborhood. They serve they who provide the paychecks.
When your interests and the interests of the city diverge, then they are terrifying.
At the same time, we cannot do without them. If the cops are not there, serious organized crime fills the vacuum. And organized crime's interests seriously diverges from ours.
Nobody has ever hit on a way to strike a good balance between necessary law enforcement and restraint. It's all culture, both within the community and within the ranks of the police. And the individual officers themselves. And that culture changes over time.