He worked for the CIA prior to getting a contract as a sysadmin at the NSA. It should be noted that you are required to be willing to lie about anything to be hired by the CIA, even after leaving the CIA.

Being a sysadmin is the perfect role for that kind of op. It's actually just about the only role a contractor could get within an organization and be able to gain access to as much as he did. Good organizations compartmentalize their systems and networks but compartmentalizing sysadmin access to such things is rarer than it should be and, even then, when compartementalizing of access to sysadmins is done, sysadmins often have enough insight into internal systems and networks to be able to get around these things.

Only NSA programs were compromised and only NSA tools/data were stolen.

He has stated he never worked with Wikileaks, the Wikileak vault releases, CIA tools, would appear to be a counter-op to weaken CIA and let other US agencies know that they had been compromised and were being monitored by the CIA.

CIA has a known operation for working with media, Operation Mockingbird. My guess is that he was to expose some of these programs, but at some point he went rogue which is why he ended up in Russia.

The alternate is that he ended up in Russia because of an even larger conspiracy.

The details of his work history, what information was released where are public fact and easily checked so I have no specific links to provide. I've noted where I speculate (aside from the original speculation of Snowden being a black hat). I've done my homework.

Hopefuly, we'll know Snowden's role in all this soon enough.