As I mentioned in another comment, one of the most amazing examples of this was the British Army/MI5 mole inside the IRA, Freddie Scappaticci, codenamed Steaknife. He was a leading member of the IRA's internal security team.
It really feels counterintuitive that someone who has got to the top would be the one to turn, but it also makes sense that they would be the one's targetted.
I have no information about this specific case, but generally speaking I think that it would be in their best interests for an intelligence agency to push their asset to the top of the org through any means in order to maximise their impact, so, at least to me, it seems plausible that many of the people at the top are informants.
It would be difficult to turn top leadership in a group because they are probably the most devoted. It's likely easier to start from the bottom. The guy that always has time to attend meetings or participate or throw in some cash (the ideal member) is going to quickly rise in the ranks. The guy with 5 kids at home and is too busy with work is not going to be an given leadership roles. The guy that only needs a fig leaf of a job is going to have plenty of time to "help" the group plan and execute tasks. He's always going to have some connect that can come through to move things forward. A fed isn't going to be the one to suggest crimes but they will certainly be there at every moment to eagerly help move it forward.
They have the most to lose personally. The rank and file can slither away, but once you’re up in the organization it’s hard to pretend that you’re an innocent.
People attracted to this stuff tend to be fairly dysfunctional as well. Even in successful revolutionary movements, the early people always get purged.
It really feels counterintuitive that someone who has got to the top would be the one to turn, but it also makes sense that they would be the one's targetted.