I respect Snowden's ability to clearly articulate his position. I look forward to his book. And unlike the somewhat polarizing discussions of others, I found his statement on the difference to between intelligence and the actions of covert action, propaganda, and influence to be quite reasonable. In much the same way that accounting systems suggest you firewall the people who can sign checks from the people who can enter legitimate expenses to prevent embezzlement, the 'rules' in an intelligence agency have to firewall the collection and analysis of intelligence from the operational abilities on the other side.
We see this is civil intelligence agencies where police officers (who are the operational end of law enforcement) are given direct access to intelligence sources (from places like Palintir) and then abuse that relationship to collect intelligence on enemies (like ex spouses) in order to inform how they can operate against them.
A more effective system is to put a strong firewall between the intelligence and the operator so that there is always documentation when operations requests intelligence information and the justification given for that collection. It creates an auditable paper trail that allows for a prosecutorial effort to be mounted against bad actors.
As one person put it, watching the watchers only works if you have a record of everything they have done that can be authenticated. Such a system would log every piece of intelligence an analyst received, and with a check in place to validate a 'need to know' for things like a target's spouses nude photos, your auditors could weed out bad actors in the pipeline. That is the sort of structure that makes institutions stable and more difficult to corrupt. Processes that survive execution with a number of bad actors.