The NSA is very explicit that their goal is to perform legal surveillance to safeguard the security of the US. Since the truth is secret, to believe either party's goals are as they say is purely a matter of trust.
The NSA has been proven to lie to its overseers repeatedly over the course of decades while Snowden has been proven to lie to the NSA in order to expose its lies. Personally, I regard Snowden's behavior to be more trustworthy in this respect.
Ah, the good old two-wrongs-make-a-right school of morality.
It's more of lesser of two evils school of morality.