I know I would be pretty bitter in a similar situation.
I think he said at one point in the IM's to Zuckerberg Greenspan said that he writes stuff to vent. And Zuckerberg said he could understood the need for that, but kept such thoughts to himself. (See e.g. Greenspan's White Paper enronforkids.pdf)
It's another thing to actively engage others, talk to them as if you are friends or enter into agreements to work together, and then take ideas from their work. It's the betrayal of people's trust that's gotten him the reputation for being a con and a thief.
None of this should be even remotely interesting until you consider hundreds of millions of people have sent their personal information to the manipulative kid in the IM's. Crazy.
He may, in fact, be the greatest con of all time, having effectively convinced an entire nation, including the President of the United States, to believe in his extremist philosophy of radical openness.
It isn't clear if getting someone to believe in your 'extremist philosophy of radical openness' is necessarily a con, unless of course you yourself don't believe in it and/or are doing so only in furtherance of some other interest which you are obscuring.
I do believe this is what Greenspan was getting at.