The CIA has used torture and overthrown governments. Since the NSA is another secretive US government agency, I have no reason to believe that there is such a thing as going too far for them in an ethical sense, although of course there are practical limits on what they can accomplish.
Also, like any organization, the NSA is not monolithic. It must have various strata, various different groups making it up. It is possible, for example, that 95% of the NSA operates in ways that are relatively ethical, but that there is also a more-or-less secret pipeline of information and control built into the organization's architecture that enables important figures both inside and outside of the organization to use its databases and tools and to coordinate its operations without the knowledge of most of its employees.
Also, like any organization, the NSA is not monolithic. It must have various strata, various different groups making it up. It is possible, for example, that 95% of the NSA operates in ways that are relatively ethical, but that there is also a more-or-less secret pipeline of information and control built into the organization's architecture that enables important figures both inside and outside of the organization to use its databases and tools and to coordinate its operations without the knowledge of most of its employees.