adversary - one that contends with, opposes, or resists

It is a matter of trust of other humans with power. Government can do good but is made of flawed humans. Trusting the government to always be good and stay good is a recipe for disaster. For government to stay trustworthy it requires people to oppose oversteps. It is an adversarial relationship. That isn't a bad thing. It is necessary for everything to work and stay working.