Should governments kill their citizens? Or any other innocent people?

Should governments kill their citizens? Or any other innocent people? We ask this question simply because governments are supposed, and they often comment they are entrusted with a duty to "protect and serve" their constituents. How can killing of any defenseless persons, by whatever means, possibly be construed as "protecting" and "serving"? Do governments use their presumed powers of life and death responsibly? And have governments allowed their duty to "protect and serve" their citizens to be neglected (or altogether denied) in place merely maintaining and expanding a government's power?