I don't think it's necessary that he "know or should have known" what he was getting into. We have shitloads of people with top secret clearance in this country. In order to view certain documents for various jobs, it's just a clearance you have to have. There isn't a requirement to know the larger picture, and where you stand, with any specific accuracy.

Do you believe it's plausible to change your views? I think the most likely situation is that he thought he knew what he was getting into, and, through his access to lots of top secret documents, ended up learning more things, and thus changed his mind.

I mean seriously, have you never gotten into a situation where you learned information that changed your mind?